Synechron Most Frequently Asked Latest SQL Server Interview Questions Answers
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.
Inner Join
Outer Join
Cross Join
Outer join again classified into 3 types.
Right Outer Join
Left Outer Join
Full Outer Join.
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.
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.
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.
Table variables don’t participate in transactions, logging or locking. This means they’re faster as they don’t require the overhead.
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.
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.
Derived tables
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 –
Recursive
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.
DateTime occupies 4 Bytes of data where as SmallDateTime occupies only 2 Bytes.
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 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:
The ability to access data from outside of SQL Server.
The ability to issue distributed queries, updates, commands, and transactions on heterogeneous data sources across the enterprise.
The ability to address diverse data sources similarly.
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.
Count returns INT datatype value where as Count_Big returns BIGINT datatype value.
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 –
Count – Select count(*) from tablename
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.
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.
Inner Join
Outer Join
Cross Join
Outer join again classified into 3 types.
Right Outer Join
Left Outer Join
Full Outer Join.
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.
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.
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.
Table variables don’t participate in transactions, logging or locking. This means they’re faster as they don’t require the overhead.
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.
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| Synechron Most Frequently Asked Latest SQL Server Interview Questions Answers |
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.
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.
Derived tables
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 –
Recursive
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.
DateTime occupies 4 Bytes of data where as SmallDateTime occupies only 2 Bytes.
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 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:
The ability to access data from outside of SQL Server.
The ability to issue distributed queries, updates, commands, and transactions on heterogeneous data sources across the enterprise.
The ability to address diverse data sources similarly.
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.
Count returns INT datatype value where as Count_Big returns BIGINT datatype value.
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 –
Count – Select count(*) from tablename
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.
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