SSIS Interview Questions and Answers

• What do we mean by dataflow in SSIS?
Data flow is nothing but the flow of data from the corresponding sources to the referred destinations. In this process, the data transformations make changes to the data to make it ready for the data warehouse.
• What is a breakpoint in SSIS? How is it setup? How do you disable it?
A breakpoint is a stopping point in the code. The breakpoint can give the Developer\DBA an
opportunity to review the status of the data, variables and the overall status of the SSIS package.
10 unique conditions exist for each breakpoint.
Breakpoints are setup in BIDS. In BIDS, navigate to the control flow interface. Right click on the
object where you want to set the breakpoint and select the ‘Edit Breakpoints…’ option.
• Can you name 5 or more of the native SSIS connection managers?
1) OLEDB connection – Used to connect to any data source requiring an OLEDB connection (i.e.,
SQL Server 2000)
2) Flat file connection – Used to make a connection to a single file in the File System. Required for reading information from a File System flat file
3) ADO.Net connection – Uses the .Net Provider to make a connection to SQL Server 2005 or other
connection exposed through managed code (like C#) in a custom task
4) Analysis Services connection – Used to make a connection to an Analysis Services database or project. Required for the Analysis Services DDL Task and Analysis Services Processing Task
5) File connection – Used to reference a file or folder. The options are to either use or create a file or folder
6) Excel
• What is the use of Bulk Insert Task in SSIS?
Bulk Insert Task is used to upload large amount of data from flat files into Sql Server. It supports only OLE DB connections for destination database.
• What is Conditional Split transformation in SSIS?
This is just like IF condition which checks for the given condition and based on the condition evaluation, the output will be sent to the appropriate OUTPUT path. It has ONE input and MANY outputs. Conditional Split transformation is used to send paths to different outputs based on some conditions. For example, we can organize the transform for the students in a class who have marks greater than 40 to one path and the students who score less than 40 to another path.
• How do you eliminate quotes from being uploaded from a flat file to SQL Server? 
This can be done using TEXT QUALIFIER property. In the SSIS package on the Flat File Connection Manager Editor, enter quotes into the Text qualifier field then preview the data to ensure the quotes are not included.
• Can you explain how to setup a checkpoint file in SSIS?
The following items need to be configured on the properties tab for SSIS package:CheckpointFileName – Specify the full path to the Checkpoint file that the package uses to save the value of package variables and log completed tasks. Rather than using a hard-coded path as shown above, it’s a good idea to use an expression that concatenates a path defined in a package variable and the package name.CheckpointUsage – Determines if/how checkpoints are used. Choose from these options: Never(default), IfExists, or Always. Never indicates that you are not using Checkpoints. IfExists is the typical setting and implements the restart at the point of failure behavior. If a Checkpoint file is found it is used to restore package variable values and restart at the point of failure. If a Checkpoint file is not found the package starts execution with the first task. The Always choice raises an error if the Checkpoint file does not exist.SaveCheckpoints – Choose from these options: True or False (default). You must select True to implement the Checkpoint behavior.
• What are the different values you can set for CheckpointUsage property ?
There are three values, which describe how a checkpoint file is used during package execution:
1) Never: The package will not use a checkpoint file and therefore will never restart.
2) If Exists: If a checkpoint file exists in the place you specified for the CheckpointFilename property, then it will be used, and the package will restart according to the checkpoints written.
3) Always: The package will always use a checkpoint file to restart, and if one does not exist, the package will fail.
• What is the ONLY Property you need to set on TASKS in order to configure CHECKPOINTS to RESTART package from failure?
The one property you have to set on the task is FailPackageOnFailure. This must be set for each task or container that you want to be the point for a checkpoint and restart. If you do not set this property to true and the task fails, no file will be written, and the next time you invoke the package, it will start from the beginning again.
• Where can we set the CHECKPOINTS, in DataFlow or ControlFlow ?
Checkpoints only happen at the Control Flow; it is not possible to checkpoint transformations or restart inside a Data Flow. The Data Flow Task can be a checkpoint, but it is treated as any other task.
• Can you explain different options for dynamic configurations in SSIS?
1) XML file
2) custom variables
3) Database per environment with the variables
4) Use a centralized database with all variables
• What is the use of Percentage Sampling transformation in SSIS?
Percentage Sampling transformation is generally used for data mining. This transformation builds a random sample of set of output rows by choosing specified percentage of input rows. For example if the input has 1000 rows and if I specify 10 as percentage sample then the transformation returns 10% of the RANDOM records from the input data.
• What is the use of Term Extraction transformation in SSIS?
Term Extraction transformation is used to extract nouns or noun phrases or both noun and noun phrases only from English text. It extracts terms from text in a transformation input column and then writes the terms to a transformation output column. It can be also used to find out the content of a dataset.
• What is Data Viewer and what are the different types of Data Viewers in SSIS?
A Data Viewer allows viewing data at a point of time at runtime. If data viewer is placed before and after the Aggregate transform, we can see data flowing to the transformation at the runtime and how it looks like after the transformation occurred. The different types of data viewers are:
1. Grid
2. Histogram
3. Scatter Plot
4. Column Chart.
• What is Ignore Failure option in SSIS?
In Ignore Failure option, the error will be ignored and the data row will be directed to continue on the next transformation. Let’s say you have some JUNK data(wrong type of data or JUNK data) flowing from source, then using this option in SSIS we can REDIRECT the junk data records to another transformation instead of FAILING the package. This helps to MOVE only valid data to destination and JUNK can be captured into separate file.
• Which are the different types of Control Flow components in SSIS?
The different types of Control Flow components are: Data Flow Tasks, SQL Server Tasks, Data Preparation Tasks, Work flow Tasks, Scripting Tasks, Analysis Services Tasks, Maintenance Tasks, Containers.
• What are containers? What are the different types of containers in SSIS?
Containers are objects that provide structures to packages and extra functionality to tasks. There are four types of containers in SSIS, they are: Foreach Loop Container, For Loop Container, Sequence Container and Task Host Container.
• What are the different types of Data flow components in SSIS?
There are 3 data flow components in SSIS.
1. Sources
2. Transformations
3. Destinations
• What are the different types of data sources available in SSIS?
There are 7 types of data sources provided by SSIS: a.) Data Reader source b.) Excel source c.) Flat file source d.) OLEDB source e.) Raw file source f.) XML source g.) Script component
• What is SSIS Designer?
It is a graphical tool for creating packages. It has 4 tabs: Control Flow, Data Flow, Event Handlers and Package Explorer.
• What is Control Flow tab?
It is the tab in SSIS designer where various Tasks can be arranged and configured. This is the tab where we provide and control the program flow of the project.
• What is Data Flow tab?
This is the tab where we do all the work related to ETL job. It is the tab in SSIS Designer where we can extract data from sources, transform the data and then load them into destinations.
• What is the function of control flow tab in SSIS?
On the control flow tab, the tasks including dataflow task, containers and precedence constraints that connect containers and tasks can be arranged and configured.
• What is the function of Event handlers tab in SSIS?
On the Event handlers tab, workflows can be configured to respond to package events.
For example, we can configure Work Flow when ANY task Failes or Stops or Starts ..
• What is the function of Package explorer tab in SSIS?
This tab provides an explorer view of the package. You can see what is happening in the package. The Package is a container at the top of the hierarchy.
• What is Solution Explorer?
It is a place in SSIS Designer where all the projects, Data Sources, Data Source Views and other miscellaneous files can be viewed and accessed for modification.
• How do we convert data type in SSIS?
The Data Conversion Transformation in SSIS converts the data type of an input column to a different data type.
• How are variables useful in ssis package?
Variables can provide communication among objects in the package. Variables can provide communication between parent and child packages. Variables can also be used in expressions and scripts. This helps in providing dynamic values to tasks.
• Explain Aggregate Transformation in SSIS?
It aggregates data, similar you do in applying TSQL functions like Group By, Min, Max, Avg, and Count. For example you get total quantity and Total line item for each product in Aggregate Transformation Editor. First you determine input columns, then output column name in Output Alias table in datagrid, and also operations for each Output Alias in Operation columns of the same datagrid. Some of operation functions listed below :
• Group By
• Average
• Count
• Count Distinct : count distinct and non null column value
• Min, Max, Sum
In Advanced tab, you can do some optimization here, such as setting up Key Scale option (low, medium, high), Count Distinct scale option (low, medium, high), Auto Extend factor and Warn On Division By Zero. If you check Warn On Division By Zero, the component will give warning instead of error. Key Scale option will optimize transformation cache to certain number of key threshold. If you set it low, optimization will target to 500,000 keys written to cache, medium can handle up to 5 million keys, and high can handle up to 25 million keys, or you can specify particular number of keys here. Default value is unspecified. Similar to number of keys for Count Distinct scale option. It is used to optimize number of distinct value written to memory, default value is unspecified. Auto Extend Factor is used when you want some portion of memory is used for this component. Default value is 25% of memory.
• Explain Audit Transformation ?
It allows you to add auditing information as required in auditing world specified by HIPPA and Sarbanes-Oxley (SOX). Auditing options that you can add to transformed data through this transformation are :
1. Execution of Instance GUID : ID of execution instance of the package
2. PackageID : ID of the package
3. PackageName
4. VersionID : GUID version of the package
5. Execution StartTime
6. MachineName
7. UserName
8. TaskName
9. TaskID : uniqueidentifier type of the data flow task that contains audit transformation.
• Explain Character Map Transformation?
It transforms some character. It gives options whether output result will override the existing column or add to new column. If you define it as new column, specify new column name. Operations available here are:
1. Uppercase
2. Lowercase
3. Byte reversal : such as from 0×1234 to 0×4321
4. Full width
5. Half width
6. Hiragana/katakana/traditional Chinese/simplified Chinese
7. Linguistic casing
• Explain Conditional split Transformation ?
It functions as if…then…else construct. It enables send input data to a satisfied conditional branch. For example you want to split product quantity between less than 500 and greater or equal to 500. You can give the conditional a name that easily identifies its purpose. Else section will be covered in Default Output Column name.
After you configure the component, it connect to subsequent transformation/destination, when connected, it pops up dialog box to let you choose which conditional options will apply to the destination transformation/destination.
• Explain Copy column Transformation?
This component simply copies a column to another new column. Just like ALIAS Column in T-Sql.
• Explain Data conversion Transformation?
This component does conversion data type, similar to TSQL function CAST or CONVERT. If you wish to convery the data from one type to another then this is the best bet. But please make sure that you have COMPATABLE data in the column.
• Explain Data Mining query Transformation?
This component does prediction on the data or fills gap on it. Some good scenarios uses this component is:
1. Take some input columns as number of children, domestic income, and marital income to predict whether someone owns a house or not.
2. Take prediction what a customer would buy based analysis buying pattern on their shopping cart.
3. Filling blank data or default values when customer doesn’t fill some items in the questionnaire.
• Explain Derived column Transformation?
Derived column creates new column or put manipulation of several columns into new column. You can directly copy existing or create a new column using more than one column also.
• Explain Merge Transformation?
Merge transformation merges two paths into single path. It is useful when you want to break out data into path that handles errors after the errors are handled, the data are merge back into downstream or you want to merge 2 data sources. It is similar with Union All transformation, but Merge has some restrictions :
1. Data should be in sorted order
2. Data type , data length and other meta data attribute must be similar before merged.
• Explain Merge Join Transformation?
Merge Join transformation will merge output from 2 inputs and doing INNER or OUTER join on the data. But if you the data come from 1 OLEDB data source, it is better you join through SQL query rather than using Merge Join transformation. Merge Join is intended to join 2 different data source.
• Explain Multicast Transformation?
This transformation sends output to multiple output paths with no conditional as Conditional Split does. Takes ONE Input and makes the COPY of data and passes the same data through many outputs. In simple Give one input and take many outputs of the same data.
• Explain Percentage and row sampling Transformations?
This transformation will take data from source and randomly sampling data. It gives you 2 outputs. First is selected data and second one is unselected data. It is used in situation where you train data mining model. These two are used to take the SAMPLE of data from the input data.
• Explain Sort Transformation?
This component will sort data, similar in TSQL command ORDER BY. Some transformations need sorted data.
• Explain Union all Transformation?
It works in opposite way to Merge transformation. It can take output from more than 2 input paths and combines into single output path.
• What r the possible locations to save SSIS package?
You can save a package wherever you want.
SQL Server
Package Store
File System
• What is a package?
A discrete executable unit of work composed of a collection of control flow and other objects, including data sources, transformations, process sequence, and rules, errors and event handling, and data destinations.
• What is a workflow in SSIS?
A workflow is a set of instructions on how to execute tasks.
(It is a set of instructions on how to execute tasks such as sessions, emails and shell commands. a workflow is created form work flow mgr.
• What is the diff between control flow Items and data flow Items?
The control flow is the highest level control process. It allows you to manage the run-time process activities of data flow and other processes within a package.
When we want to extract, transform and load data within a package. You add an SSIS dataflow task to the package control flow.
• What are the main component of SSIS(project-architecture)?
SSIS archItecture has 4 main components
1.ssis service
2.ssis runtime engine & runtime executables
3.ssis dataflow engine & dataflow components
4.ssis clients
• Different components in SSIS package?
Control flow
Data flow
Event handler
Package explorer
• What are Connection Managers?
It is a bridge b/w package object and physical data. It provides logical representation of a connection at design time the properties of the connection mgr describes the physical connection that integration services creates when the package is run.
• What is environment variable in SSIS?
An environment variable configuration sets a package property equal to the value in an environment variable.
Environmental configurations are useful for configuring properties that are dependent on the computer that is executing the package.
• How to provide securIty to packages?
We can provide security in two ways
1. Package encryption
2. Password protection.
• What are Precedence constraints?
Constraints that link executable, container, and tasks wIthin the package control flow and specify condItion that determine the sequence and condItions for determine whether executable run.
• What is Design time Deployment in SSIS ?
When you run a package from with in BIDS,it is built and temporarily deployed to the folder. By default the package will be deployed to the BIN folder in the Package’s Project folder and you can configure for custom folder for deployment. When the Package’s execution is completed and stopped in BIDS,the deployed package will be deleted and this is called as Design Time Deployment.

Sql Server Interview Questions with Answers

•  What are Constraints  or Define Constraints ?

Generally we use Data Types to limit the kind of Data in a Column. For example, if we declare any column with data type INT then ONLY Integer data can be inserted into the column. Constraint will help us to limit the Values we are passing into a column or a table. In simple Constraints are nothing but Rules or Conditions applied on columns or tables to restrict the data.

• Different types of Constraints ?

There are THREE Types of Constraints.

1. Domain
2. Entity
3. Referential

Domain has the following constraints types -

1. Not Null
2. Check

Entity has the following constraint types -

1. Primary Key
2. Unique Key

Referential has the following constraint types -

1. Foreign Key

• What is the difference between Primary Key and Unique Key ?

Both the Primary Key(PK) and Unique Key(UK) are meant to provide Uniqueness to the Column on which they are defined. PFB the major differences between these two.

1. By default PK defines Clustered Index in the column where as UK defines Non Clustered Index.
2. PK doesn’t allow NULL Value where as UK allow ONLY ONE NULL.
3. You can have only one PK per table where as UK can be more than one per table.
4. PK can be used in Foreign Key relationships where as UK cannot be used.

• What is the difference between Delete and Truncate ?

Both Delete and Truncate commands are meant to remove rows from a table. There are many differences between these two and pfb the same.

1. Truncate is Faster where as Delete is Slow process.
2. Truncate doesn’t log where as Delete logs an entry for every record deleted in Transaction Log.
3. We can rollback the Deleted data where as Truncated data cannot be rolled back.
4. Truncate resets the Identity column where as Delete doesn’t.
5. We can have WHERE Clause for delete where as for Truncate we cannot have WHERE Clause.
6. Delete Activates TRIGGER where as TRUNCATE Cannot.
7. Truncate is a DDL statement where as Delete is DML statement.

• What are Indexes or Indices ?

An Index in SQL is similar to the Index in a  book. Index of a book makes the reader to go to the desired page or topic easily and Index in SQL helps in retrieving the data faster from database. An Index is a seperate physical data structure that enables queries to pull the data fast. Indexes or Indices are used to improve the performance of a query.

• Types of Indices in SQL ?

There are TWO types of Indices in SQL server.

1. Clustered
2. Non Clustered

• How many Clustered and Non Clustered Indexes can be defined for a table ?

Clustered – 1
Non Clustered – 999

MSDN reference – Click Here

• What is Transaction in SQL Server ? 

Transaction groups a set of T-Sql Statements into a single execution unit. Each transaction begins with a specific task and ends when all the tasks in the group successfully complete. If any of the tasks fails, the transaction fails. Therefore, atransaction has only two results: success or failure. Incomplete steps result in the failure of the transaction.by programmers to group together read and write operations. In Simple Either FULL or NULL i.e either all the statements executes successfully or all the execution will be rolled back.

• Types of Transactions ?

There are TWO forms of Transactions.

1. Implicit – Specifies any Single Insert,Update or Delete statement as Transaction Unit.  No need to specify Explicitly.
2. Explicit – A group of T-Sql statements with the beginning and ending marked with Begin Transaction,Commit and RollBack. PFB an Example for Explicit transactions.

BEGIN TRANSACTION
Update Employee Set Emp_ID = 54321 where Emp_ID = 12345
If(@@Error <>0)
ROLLBACK
Update LEave_Details Set Emp_ID = 54321 where Emp_ID = 12345
If(@@Error <>0)
ROLLBACK
COMMIT
In the above example we are trying to update an EMPLOYEE ID from 12345 to 54321 in both the master table “Employee” and Transaction table “Leave_Details”. In this case either BOTH the tables will be updated with new EMPID or NONE.

• What is the Max size and Max number of columns for a row in a table ?

Size – 8060 Bytes

Columns – 1024

• What is Normalization and Explain different normal forms.

Database normalization is a process of data design and organization which applies to data structures based on rules that help building relational databases.
1. Organizing data to minimize redundancy.
2. Isolate data so that additions, deletions, and modifications of a field can be made in just one table and then propagated through the rest of the database via the defined relationships.
1NF: Eliminate Repeating Groups
Each set of related attributes should be in separate table, and give each table a primary key. Each field contains at most one value from its attribute domain.
2NF: Eliminate Redundant Data
1. Table must be in 1NF.
2. All fields are dependent on the whole of the primary key, or a relation is in 2NF if it is in 1NF and every non-key attribute is fully dependent on each candidate key of the relation. If an attribute depends on only part of a multi‐valued key, remove it to a separate table.
3NF: Eliminate Columns Not Dependent On Key
1. The table must be in 2NF.
2. Transitive dependencies must be eliminated. All attributes must rely only on the primary key. If attributes do not contribute to a description of the key, remove them to a separate table. All attributes must be directly dependent on the primary key.
BCNF: Boyce‐Codd Normal Form
for every one of its non-trivial functional dependencies X → Y, X is a superkey—that is, X is either a candidate key or a superset thereof. If there are non‐trivial dependencies between candidate key attributes, separate them out into distinct tables.
4NF: Isolate Independent Multiple Relationships
No table may contain two or more 1:n or n:m relationships that are not directly related.
For example, if you can have two phone numbers values and two email address values, then you should not have them in the same table.
5NF: Isolate Semantically Related Multiple Relationships
A 4NF table is said to be in the 5NF if and only if every join dependency in it is implied by the candidate keys. There may be practical constrains on information that justify separating logically related many‐to‐many relationships.

• What is Denormalization ?

For optimizing the performance of a database by adding redundant data or by grouping data is called de-normalization.
It is sometimes necessary because current DBMSs implement the relational model poorly.
In some cases, de-normalization helps cover up the inefficiencies inherent in relational database software. A relational normalized database imposes a heavy access load over physical storage of data even if it is well tuned for high performance.
A true relational DBMS would allow for a fully normalized database at the logical level, while providing physical storage of data that is tuned for high performance. De‐normalization is a technique to move from higher to lower normal forms of database modeling in order to speed up database access.

• Query to Pull ONLY duplicate records from table ?

There are many ways of doing the same and let me explain one here. We can acheive this by using the keywords GROUP and HAVING. The following query will extract duplicate records from a specific column of a particular table.
Select specificColumn
FROM particluarTable
GROUP BY specificColumn
HAVING COUNT(*) > 1
This will list all the records that are repeated in the column specified by “specificColumn” of a “particlarTable”.

• Types of Joins in SQL SERVER ?

There are 3 types of joins in Sql server.

1. Inner Join
2. Outer Join
3. Cross Join

Outer join again classified into 3 types.

1. Right Outer Join
2. Left Outer Join
3. Full Outer Join.

• What is Table Expressions in Sql Server ?

Table Expressions are subqueries that are used where a TABLE is Expected. There are TWO types of table Expressions.

1. Derived tables
2. Common Table Expressions.

• What is Derived Table ?

Derived tables are table expression which appears in FROM Clause of a Query. PFB an example of the same.

select * from (Select Month(date) as Month,Year(Date) as Year from table1) AS Table2

In the above query the subquery in FROM Clause “(Select Month(date) as Month,Year(Date) as Year from table1) ” is called Derived Table.

• What is CTE or Common Table Expression ?

Common table expression (CTE) is a temporary named result set that you can reference within a

SELECT, INSERT, UPDATE, or DELETE statement. You can also use a CTE in a CREATE VIEW statement, as part of the view’s SELECT query. In addition, as of SQL Server 2008, you can add a CTE to the new MERGE statement. There are TWO types of CTEs in Sql Server -

1. Recursive
2. Non Recursive

• Difference between SmallDateTime and DateTime datatypes in Sql server ?

Both the data types are meant to specify date and time but these two has slight differences and pfb the same.

1. DateTime occupies 4 Bytes of data where as SmallDateTime occupies only 2 Bytes.
2. DateTime ranges from 01/01/1753 to 12/31/9999 where as SmallDateTime ranges from 01/01/1900 to 06/06/2079.

• What is SQL_VARIANT Datatype ? 

The SQL_VARIANT data type can be used to store values of various data types at the same time, such as numeric values, strings, and date values. (The only types of values that cannot be stored are TIMESTAMP values.) Each value of an SQL_VARIANT column has two parts: the data value and the information that describes the value. (This information contains all properties of the actual data type of the value, such as length, scale, and precision.)

• What is Temporary table ? 

A temporary table is a database object that is temporarily stored and managed by the database system. There are two types of Temp tables.

1. Local
2. Global

• What are the differences between Local Temp table and Global Temp table ? 

Before going to the differences, let’s see the similarities.

1. Both are stored in tempdb database.
2. Both will be cleared once the connection,which is used to create the table, is closed.
3. Both are meant to store data temporarily.

PFB the differences between these two.

1. Local temp table is prefixed with # where as Global temp table with ##.
2. Local temp table is valid for the current connection i.e the connection where it is created where as Global temp table is valid for all the connection.
3.  Local temp table cannot be shared between multiple users where as Global temp table can be shared.

• Whar are the differences between Temp table and Table variable ?

This is very routine question in interviews. Let’s see the major differences between these two.

1. Table variables are Transaction neutral where as Temp tables are Transaction bound. For example if we declare and load data into a temp table and table variable in a transaction and if the transaction is ROLLEDBACK, still the table variable will have the data loaded where as Temp table will not be available as the transaction is rolled back.
2. Temporary Tables are real tables so you can do things like CREATE INDEXes, etc. If you have large amounts of data for which accessing by index will be faster then temporary tables are a good option.
3. Table variables don’t participate in transactions, logging or locking. This means they’re faster as they don’t require the overhead.
4. You can create a temp table using SELECT INTO, which can be quicker to write (good for ad-hoc querying) and may allow you to deal with changing datatypes over time, since you don’t need to define your temp table structure upfront.

• What is the difference between Char,Varchar and nVarchar datatypes ?

char[(n)] - Fixed-length non-Unicode character data with length of n bytes. n must be a value from 1 through 8,000. Storage size is n bytes. The SQL-92 synonym for char is character.
varchar[(n)] - Variable-length non-Unicode character data with length of n bytes. n must be a value from 1 through 8,000. Storage size is the actual length in bytes of the data entered, not n bytes. The data entered can be 0 characters in length. The SQL-92 synonyms for varchar are char varying or character varying.
nvarchar(n) - Variable-length Unicode character data of n characters. n must be a value from 1 through 4,000. Storage size, in bytes, is two times the number of characters entered. The data entered can be 0 characters in length. The SQL-92 synonyms for nvarchar are national char varying and national character varying.

• What is the difference between STUFF and REPLACE functions in Sql server ?

The Stuff function is used to replace characters in a string. This function can be used to delete a certain length of the string and replace it with a new string.

Syntax – STUFF (string_expression, start, length, replacement_characters)

Ex - SELECT STUFF(‘I am a bad boy’,8,3,’good’)

Output – “I am a good boy”

REPLACE function replaces all occurrences of the second given string expression in the first string expression with a third expression.

Syntax - REPLACE (String, StringToReplace, StringTobeReplaced)

Ex – REPLACE(“Roopesh”,”pe”,”ep”)

Output – “Rooepsh” – You can see PE is replaced with EP in the output.

• What are Magic Tables ? 

Sometimes we need to know about the data which is being inserted/deleted by triggers in database. Whenever a trigger fires in response to the INSERT, DELETE, or UPDATE statement, two special tables are created. These are the inserted and the deleted tables. They are also referred to as the magic tables. These are the conceptual tables and are similar in structure to the table on which trigger is defined (the trigger table).

The inserted table contains a copy of all records that are inserted in the trigger table.
The deleted table contains all records that have been deleted from deleted from the trigger table.
Whenever any updation takes place, the trigger uses both the inserted and deleted tables.

• Explain about RANK,ROW_NUMBER and DENSE_RANK in Sql server ?

Found a very interesting explanation for the same in the url Click Here . PFB the content of the same here.
Lets take 1 simple example to understand the difference between 3.
First lets create some sample data :
– create table
CREATE TABLE Salaries
(
Names VARCHAR(1),
SalarY INT
)
GO
– insert data
INSERT INTO Salaries SELECT
‘A’,5000 UNION ALL SELECT
‘B’,5000 UNION ALL SELECT
‘C’,3000 UNION ALL SELECT
‘D’,4000 UNION ALL SELECT
‘E’,6000 UNION ALL SELECT
‘F’,10000
GO
– Test the data
SELECT Names, Salary
FROM Salaries
Now lets query the table to get the salaries of all employees with their salary in descending order.
For that I’ll write a query like this :
SELECT names
, salary
,row_number () OVER (ORDER BY salary DESC) as ROW_NUMBER
,rank () OVER (ORDER BY salary DESC) as RANK
,dense_rank () OVER (ORDER BY salary DESC) as DENSE_RANK
FROM salaries
>>Output

NAMES	SALARY	ROW_NUMBER	RANK	DENSE_RANK
F	10000	1	1	1
E	6000	2	2	2
A	5000	3	3	3
B	5000	4	3	3
D	4000	5	5	4
C	3000	6	6	5

Interesting Names in the result are employee A, B and D.  Row_number assign different number to them. Rank and Dense_rank both assign same rank to A and B. But interesting thing is what RANK and DENSE_RANK assign to next row? Rank assign 5 to the next row, while dense_rank assign 4.
The numbers returned by the DENSE_RANK function do not have gaps and always have consecutive ranks.  The RANK function does not always return consecutive integers.  The ORDER BY clause determines the sequence in which the rows are assigned their unique ROW_NUMBER within a specified partition.
So question is which one to use?
Its all depends on your requirement and business rule you are following.
1. Row_number to be used only when you just want to have serial number on result set. It is not as intelligent as RANK and DENSE_RANK.
2. Choice between RANK and DENSE_RANK depends on business rule you are following. Rank leaves the gaps between number when it sees common values in 2 or more rows. DENSE_RANK don’t leave any gaps between ranks.
So while assigning the next rank to the row RANK will consider the total count of rows before that row and DESNE_RANK will just give next rank according to the value.
So If you are selecting employee’s rank according to their salaries you should be using DENSE_RANK and if you are ranking students according to there marks you should be using RANK(Though it is not mandatory, depends on your requirement.)

•  What are the differences between WHERE and HAVING clauses in SQl Server ? 

PFB the major differences between WHERE and HAVING Clauses ..

1.Where Clause can be used other than Select statement also where as Having is used only with the SELECT statement.
2.Where applies to each and single row and Having applies to summarized rows (summarized with GROUP BY).
3.In Where clause the data that fetched from memory according to condition and In having the completed data firstly fetched and then separated according to condition.
4.Where is used before GROUP BY clause and HAVING clause is used to impose condition on GROUP Function and is used after GROUP BY clause in the query.

• Explain Physical Data Model or PDM ?

Physical data model represents how the model will be built in the database. A physical database model shows all table structures, including column name, column data type, column constraints, primary key, foreign key, and relationships between tables. Features of a physical data model include:

1. Specification all tables and columns.
2. Foreign keys are used to identify relationships between tables.
3. Specying Data types.

EG -

Reference from Here

•  Explain Logical Data Model ?

A logical data model describes the data in as much detail as possible, without regard to how they will be physical implemented in the database. Features of a logical data model include:

1. Includes all entities and relationships among them.
2. All attributes for each entity are specified.
3. The primary key for each entity is specified.
4. Foreign keys (keys identifying the relationship between different entities) are specified.
5. Normalization occurs at this level.

Reference from Here

• Explain Conceptual Data Model ?

A conceptual data model identifies the highest-level relationships between the different entities. Features of conceptual data model include:

1. Includes the important entities and the relationships among them.
2. No attribute is specified.
3. No primary key is specified.

Reference from Here

• What is Log Shipping ?

Log Shipping is a basic level SQL Server high-availability technology that is part of SQL Server. It is an automated backup/restore process that allows you to create another copy of your database for failover.
Log shipping involves copying a database backup and subsequent transaction log backups from the primary (source) server and restoring the database and transaction log backups on one or more secondary (Stand By / Destination) servers. The Target Database is in a standby or no-recovery mode on the secondary server(s) which allows subsequent transaction logs to be backed up on the primary and shipped (or copied) to the secondary servers and then applied (restored) there.

•  What are the advantages of database normalization ?

Benefits of normalizing the database are

1. No need to restructure existing tables for new data.
2. Reducing repetitive entries.
3. Reducing required storage space
4. Increased speed and flexibility of queries.

•  What are Linked Servers  ?

 Linked servers are configured to enable the Database Engine to execute a Transact-SQL statement that includes tables in another instance of SQL Server, or another database product such as Oracle. Many types OLE DB data sources can be configured as linked servers, including Microsoft Access and Excel. Linked servers offer the following advantages:

1. The ability to access data from outside of SQL Server.
2. The ability to issue distributed queries, updates, commands, and transactions on heterogeneous data sources across the enterprise.
3. The ability to address diverse data sources similarly.
4. Can connect to MOLAP databases too.

•  What is the Difference between the functions COUNT and COUNT_BIG ?

Both Count and Count_Big functions are used to count the number of rows in a table and the only difference is what it returns.

1. Count returns INT datatype value where as Count_Big returns BIGINT datatype value.
2. Count is used if the rows in a table are less where as Count_Big will be used when the numbenr of records are in millions or above.

Syntax -

1. Count – Select count(*) from tablename
2. Count_Big – Select Count_Big(*) from tablename

•  How to insert values EXPLICITLY  to an Identity Column ? 

This has become a common question these days in interviews. Actually we cannot Pass values to Identity column and you will get the following error message when you try to pass value.

Msg 544, Level 16, State 1, Line 3
Cannot insert explicit value for identity column in table 'tablename' when IDENTITY_INSERT is set to OFF.

To pass an external value we can use the property IDENTITY_INSERT. PFB the sysntax of the same.

SET IDENTITY_INSERT <tablename> ON;

Write your Insert  statement here by passing external values to the IDENTITY column.

Once the data is inserted then remember to SET the property to OFF.

• How to RENAME a table and column in SQL ?

We can rename a table or a column in SQL using the System stored procedure SP_RENAME. PFB the sample queries.

Table - EXEC sp_rename @objname = department, @newname = subdivision

Column - EXEC sp_rename @objname = ‘sales.order_no’ , @newname = ordernumber

• How to rename a database ?

To rename a database please use the below syntax.

USE master;
GO
ALTER DATABASE databasename
Modify Name = newname ;
GO

•  What is the use the UPDATE_STATISTICS command ?

UPDATE_STATISTICS updates the indexes on the tables when there is large processing of data. If we do a large amount of deletions any modification or Bulk Copy into the tables, we need to basically update the indexes to take these changes into account.

• How to read the last record from a table with Identity Column ?

We can get the same using couple of ways and PFB the same.

First - 

SELECT *
FROM    TABLE
WHERE  ID = IDENT_CURRENT(‘TABLE’)

Second - 

SELECT *
FROM    TABLE
WHERE   ID = (SELECT MAX(ID)  FROM TABLE)

Third - 

select top 1 * from TABLE_NAME  order by ID desc

• What is Worktable ?

A worktable is a temporary table used internally by SQL Server to process the intermediate results of a query. Worktables are created in the tempdb database and are dropped automatically after query execution. Thease table cannot be seen as these are created while a query executing and dropped immediately after the execution of the query.

• What is HEAP table ?

A table with NO CLUSTERED INDEXES is called as HEAP table. The data rows of a heap table are not stored in any particular order or linked to the adjacent pages in the table. This unorganized structure of the heap table usually increases the overhead of accessing a large heap table, when compared to accessing a large nonheap table (a table with clustered index). So, prefer not to go with HEAP  tables ..

• What is ROW LOCATOR ?

If you define a NON CLUSTERED index on a table then the index row of a nonclustered index contains a pointer to the corresponding data row of the table. This pointer is called a row locator. The value of the row locator depends on whether the data pages are stored in a heap or are clustered. For a nonclustered index, the row locator is a pointer to the data row. For a table with a clustered index, the row locator is the clustered index key value.

•  What is Covering Index ?

A covering index is a nonclustered index built upon all the columns required to satisfy a SQL query without going to the base table. If a query encounters an index and does not need to refer to the underlying data table at all, then the index can be considered a covering index.  For Example
Select col1,col2 from table
where col3 = Value
group by col4
order by col5
Now if you create a clustered index for all the columns used in Select statement then the SQL doesn’t need to go to base tables as everything required are available in index pages.

•  What is Indexed View ?

A database view in SQL Server is like a virtual table that represents the output of a SELECT statement. A view is created using the CREATE VIEW statement, and it can be queried exactly like a table. In general, a view doesn’t store any data—only the SELECT statement associated with it. Every time a view is queried, it further queries the underlying tables by executing its associated SELECT statement.

A database view can be materialized on the disk by creating a unique clustered index on the view. Such a view is referred to as an indexed view. After a unique clustered index is created on the view, the view’s result set is materialized immediately and persisted in physical storage in the database, saving the overhead of performing costly operations during query execution. After the view is materialized, multiple nonclustered indexes can be created on the indexed view.

• What is Bookmark Lookup ?

When a SQL query requests a small number of rows, the optimizer can use the nonclustered index, if available, on the column(s) in the WHERE clause to retrieve the data. If the query refers to columns that are not part of the nonclustered index used to retrieve the data, then navigation is required from the index row to the corresponding data row in the table to access these columns.This operation is called a bookmark lookup.