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Interview Questions and Answers
What is a NOLOCK?
Using the NOLOCK query optimiser hint is generally considered good practice in order to improve concurrency on a busy system. When the NOLOCK hint is included in a SELECT statement, no locks are taken when data is read. The result is a Dirty Read, which means that another process could be updating the data at the exact time you are reading it. There are no guarantees that your query will retrieve the most recent data. The advantage to performance is that your reading of data will not block updates from taking place, and updates will not block your reading of data. SELECT statements take Shared (Read) locks. This means that multiple SELECT statements are allowed simultaneous access, but other processes are blocked from modifying the data. The updates will queue until all the reads have completed, and reads requested after the update will wait for the updates to complete. The result to your system is delay(blocking).
What is difference between DELETE & TRUNCATE commands?
Delete command removes the rows from a table based on the condition that we provide with a WHERE clause. Truncate will actually remove all the rows from a table and there will be no data in the table after we run the truncate command.
TRUNCATE
TRUNCATE is faster and uses fewer system and transaction log resources than DELETE.
TRUNCATE removes the data by deallocating the data pages used to store the table’s data, and only the page deallocations are recorded in the transaction log.
TRUNCATE removes all rows from a table, but the table structure and its columns, constraints, indexes and so on remain. The counter used by an identity for new rows is reset to the seed for the column.
You cannot use TRUNCATE TABLE on a table referenced by a FOREIGN KEY constraint.
Because TRUNCATE TABLE is not logged, it cannot activate a trigger.
TRUNCATE can not be Rolled back using logs.
TRUNCATE is DDL Command.
TRUNCATE Resets identity of the table.
DELETE
DELETE removes rows one at a time and records an entry in the transaction log for each deleted row.
If you want to retain the identity counter, use DELETE instead. If you want to remove table definition and its data, use the DROP TABLE statement.
DELETE Can be used with or without a WHERE clause
DELETE Activates Triggers.
DELETE Can be Rolled back using logs.
DELETE is DML Command.
DELETE does not reset identity of the table.
Difference between Function and Stored Procedure?
UDF can be used in the SQL statements anywhere in the WHERE/HAVING/SELECT section where as Stored procedures cannot be.
UDFs that return tables can be treated as another rowset. This can be used in JOINs with other tables.
Inline UDF’s can be though of as views that take parameters and can be used in JOINs and other Rowset operations.
When is the use of UPDATE_STATISTICS command?
This command is basically used when a large processing of data has occurred. If a large amount of deletions any modification or Bulk Copy into the tables has occurred, it has to update the indexes to take these changes into account. UPDATE_STATISTICS updates the indexes on these tables accordingly.
What types of Joins are possible with Sql Server?
Joins are used in queries to explain how different tables are related. Joins also let you select data from a table depending upon data from another table.
Types of joins: INNER JOINs, OUTER JOINs, CROSS JOINs. OUTER JOINs are further classified as LEFT OUTER JOINS, RIGHT OUTER JOINS and FULL OUTER JOINS.
What is the difference between a HAVING CLAUSE and a WHERE CLAUSE?
Specifies a search condition for a group or an aggregate. HAVING can be used only with the SELECT statement. HAVING is typically used in a GROUP BY clause. When GROUP BY is not used, HAVING behaves like a WHERE clause. Having Clause is basically used only with the GROUP BY function in a query. WHERE Clause is applied to each row before they are part of the GROUP BY function in a query. HAVING criteria is applied after the the grouping of rows has occurred.
What is sub-query? Explain properties of sub-query.
Sub-queries are often referred to as sub-selects, as they allow a SELECT statement to be executed arbitrarily within the body of another SQL statement. A sub-query is executed by enclosing it in a set of parentheses. Sub-queries are generally used to return a single row as an atomic value, though they may be used to compare values against multiple rows with the IN keyword.
A subquery is a SELECT statement that is nested within another T-SQL statement. A subquery SELECT statement if executed independently of the T-SQL statement, in which it is nested, will return a result set. Meaning a subquery SELECT statement can standalone and is not depended on the statement in which it is nested. A subquery SELECT statement can return any number of values, and can be found in, the column list of a SELECT statement, a FROM, GROUP BY, HAVING, and/or ORDER BY clauses of a T-SQL statement. A Subquery can also be used as a parameter to a function call. Basically a subquery can be used anywhere an expression can be used.
Properties of Sub-Query
A subquery must be enclosed in the parenthesis.
A subquery must be put in the right hand of the comparison operator, and
A subquery cannot contain a ORDER-BY clause.
A query can contain more than one sub-queries.
What are types of sub-queries?
Single-row subquery, where the subquery returns only one row.
Multiple-row subquery, where the subquery returns multiple rows,.and
Multiple column subquery, where the subquery returns multiple columns.
Complete Series of SQL Server Interview Questions and Answers
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SQL Server Interview Questions and Answers – Part 1
SQL Server Interview Questions and Answers – Part 2
SQL Server Interview Questions and Answers – Part 3
SQL Server Interview Questions and Answers – Part 4
SQL Server Interview Questions and Answers – Part 5
SQL Server Interview Questions and Answers – Part 6
SQL Server Interview Questions and Answers Complete List Download
Reference : Pinal Dave (https://blog.sqlauthority.com)
42 Comments.
Why and How to use Create Statistic command?
Hi
you have mentioned that trucnate table cannot be rolled back.
Inside a transaction on truncating a table the dellocation of the data pages is logged and hence truncation can be rolled back .
hi,
what are the commands which are not allowed in SQL Server Triggers.
Regards
please solve my all queries which is listed below
To find the 5th row of a table.
To find 2nd highest salary
To find 3 highest salary
To delete duplicate records of a table.
Group functions
Types of join, describe self join and equi join
special operators
Delete records where ename is duplicated
To find 10th lowest salary
union operator
please solve my all queries which is listed below
To find the 5th row of a table.
To find 2nd highest salary
To find 3 highest salary
To delete duplicate records of a table.
Group functions
Types of join, describe self join and equi join
special operators
Delete records where ename is duplicated
To find 10th lowest salary
union operator
I don’t agree your statement that “TRUNCATE cannot be rolled back”.
This example demonstrates that a TRUNCATE statement can be rolled back in an explicit transaction.
create table test (id int identity(1,1), f int)
declare @i int
set @i = 100
while @i>0
BEGIN
set @i = @i-1
insert into test(f)values(@i)
end
— the table contains 100 rows
select * from test
begin tran
truncate table test
select * from test
— now the table is empty
rollback
— the table has again 100 rows
select * from test
It should be stressed that TRUNCATE is a DDL statement! From BOL: “TRUNCATE TABLE permissions default to the table owner, members of the sysadmin fixed server role, and the db_owner and db_ddladmin fixed database roles, and are not transferable.”
How to eliminate duplicate rows in a table
Can u tell me the difference between select and choose statement in SQL?
Hi,
I need to delete a row with primary key = 17
but wen I reenter informatio into the table it displays 16,18 the deleleted.
How to delete but retain the identity
Sir
Could you clear my doubt regarding the possibility of TRUNCATE being rolled back?Plz get me a solid answer.I could find “TRUNCATE cannot be rolled back” statement in some resources.At the same time,some other resources say “Ofcourse TRUNCATE can be rolled back”.Which is the correct one?Iam Confused..Plz help me out
Hi srikanth,
we can delete the duplicate data by use of distinct keyword
Hi pinaldev ,
can you give me a brief information of what is a trigger,its use,
how it works and in what situation we use.
thanks,
Pavanich
how can we get the maximum salary from a table without using max keyword in oracle
Hi pinaldev ,
can you give me a brief information of what is a trigger,its use,the types,and how it works and in what situation we use.
please explain me with an example
thanks,
Pavanich
How to avoid Cursors?
Colud you please explain .. How to interprete the Query Excecution plan to improve the performance
Dear Pinal,
just to add some differences between a sp and a table :
1. when you add any exec statement which executes sps or any dynamic query in a fn , you will get this msg from the execution engine :
Only functions and extended stored procedures can be executed from within a function.
2. you cannot access temporary tables from within a function.
Regards,
Abhay
Hi,
Suppose I have a table of all the salaries of employees and want to to find out the 5th highest record in the table then what will the query be like?
Thanks
Robert Vaz
select * from t1 where n= (select count(*) from t2 where t1.[ColumnName] <=t2.[ColumnName])
this is generailzed Query for Finding Nth highest Records Just replace n for the position.
Hi
pavanich i saw your sql server interview questions its really good.I have one problem can you please help me.
The details are
i have 4 tables named as A,B,C,D
the table A has two columns EMPID,REFNo.
In The remaining all tables EMPID Column is there.this is common column in all the tables
BUt I will authenticate the user by using REFNo.
Depending on that REFNO i have to get the EMPID From A table . comparing EMPID in the remaining all 3 tables and
want to get the data from these 3 tables .
please help me