Skip to main content

Interview Questions and answers - SQL Server Day 5

Question: What is difference between Truncate and Delete in SQL Server?

Answer: If you wish to removes rows basis on condition then we have to use DELETE command while truncate will remove all the rows from a table.

TRUNCATE
  • Truncate is DDL command.
  • Truncate command removes all data from the table.
  • Truncate does not keep the logs so it fast then DELETE.
  • Truncate statement is not conditional mean we cannot use WHERE clause in truncate statement.
  • Rollback is not possible.
  • Truncate reset the table to its empty state.

DELETE
  • Delete is a DML command
  • We can use WHERE clause in delete statement.
  • Delete statement delete specific rows if where condition exist.
  • Delete command lock the row when executed, each row in table is locked for deletion.
  • Delete operation are logged individually so delete activates the trigger.
  • Rollback is possible in delete.
  • Delete keeps log so delete is slower then truncate.

Question: What are DML, DDL, DCL and TCL in SQL- Server?

Answer: 
DML is stand for Data Manipulation Language. DML is responsible to retrieve, store, modify, delete, insert and update data in database. Examples: SELECT, UPDATE, and INSERT statements.

DDL is stand for Data Definition Language. DDL is responsible to create and modify the structure of database objects in database. Examples: CREATE, ALTER, and DROP statements.

DCL is stand for Data Control Language. DCL is responsible to roles, permissions, and referential integrity and also use to control access to database by securing it. Examples: GRANT, REVOKE statements

TCL is stand for Transactional Control Language. TCL is responsible to manage different transactions occurring within a database. Examples: COMMIT, ROLLBACK statements.

Question: Write query SELECT, INSERT, DELETE and UPDATE Statements in SQL Server?

Answer: Suppose we have a table Employee with column 'FNAME', 'LNAME', 'ADDRESS' and CREATEDDATE

SELECT Statement: SELECT * FROM Employee

INSERT Statement:

INSERT INTO Employee (FNAME,LNAME,ADDRESS,CREATEDDATE) VALUES('DILIP','SINGH','DELHI',GETDATE())
INSERT INTO Employee (FNAME,LNAME,ADDRESS,CREATEDDATE) VALUES('VIPUL','BHATT','NOIDA',GETDATE())
INSERT INTO Employee (FNAME,LNAME,ADDRESS,CREATEDDATE) VALUES('RAJ','SINGH','BOKARO',GETDATE())

DELETE Statement: 

DELETE FROM Employee

Note: this query will delete all records of table.

Delete records with condition, where clause use for condition
DELETE FROM Employee WHERE FNAME='DILIP'

UPDATE Statement:

UPDATE Employee SET ADDRESS='GORAKHPUR'
Note: It will update ADDRESS column with 'GORKHPUR' of all table row.

Update Statement with condition
UPDATE Employee SET ADDRESS='GORAKHPUR' WHERE FNAME='DILIP'

Question: What is difference between a HAVING clause and a WHERE clause in SQL Server?

Answer: Only with the SELECT statement we can be use HAVING Clause.  Basically HAVING Clause is used with GROUP BY Clause. Without used of GROUP BY clause, HAVING behaves like a WHERE clause.

Where Clause Example: SELECT empName,empDept,empSalary WHERE empSalary=20000

HAVING Clause Example: SELECT empName,AVG(empSalary) GROUP BY empName,empSalary HAVING AVG(empSalary)>20000
For more click here 

Question: What is Difference between Primary key and Unique Key in SQL Server?

Answer:
  1. Primary key doesn’t have null value while unique key can hold null value.
  2. A table can have only one primary key while a table can have more than one unique key
  3. By default primary key have clustered index while by default unique key have non-clustered index


Question: What are difference between @@IDENTITY, SELECT SCOPE_IDENTITY(), SELECT IDENT_CURRENT in SQL Server?

Answer:
SELECT @@IDENTITY: it’s responsible to returns the last identity value generated for any table in the current session, across all scopes (i.e. global scope).

SELECT SCOPE_IDENTITY(): It’s responsible to returns the last identity value generated for any table in the current session and the current scope(i.e. local scope).

SELECT IDENT_CURRENT(‘table_name’): It’s responsible to returns the last identity value generated for a specific table in any session and any scope (i.e. global scope). For more with example click here 

Question: How to get first & last day of the previous Week in SQL Server?

Answer:  Following query may help you
This query will return first day of the previous Week
SELECT CONVERT(DATE,DATEADD(WK, DATEDIFF(WK, 0, GETDATE()) - 1, 0)) First_Day_Of_Prev_Week

This query wil return last day of the previous Week

SELECT CONVERT(DATE,DATEADD(WK, DATEDIFF(WK, 0, GETDATE()) - 1, 0) + 6) Last_Day_OF_Prev_Week

Popular posts from this blog

Query to find stored procedures by nested stored procedure name

Problem: Suppose we have a stored procedure which has been used in several stored procedure, I mean stored procedure usp_proc1 is nested in many stored procedures like below
BEGIN DECLARE@ResultTABLE ( IDINT, NAMEVARCHAR(50), [ADDRESS]VARCHAR(255) ) INSERTINTO@Result EXECusp_proc1@Name='codefari' END
So I want to find all those queries who containing usp_proc1
Solution: There is a lot of solutions, I'm giving some of them below. If you want to get the only name of the stored procedures then use the following query. Using join query on system tables syscomments and sysobjects we can get the stored procedures name which containing the particular table, nested procs or any other string.
SELECTDISTINCTo.name FROMsyscommentss INNERJOINsysobjectsoONs.id=o.id

Check for changes to an SQL Server table?

Problem: Suppose your team is working on the under-development project so it might be possible continuous work on the database and perform changes in Table, Stored procedure as per requirement, and daily you have to update the testing server database as per changes are done in developing server database then how it is possible to trace those changes. There are a lot of solutions for this problem which is listed below Solution 1: For SQL Server 2000, 2005 and above use the CHECKSUM command SELECTCHECKSUM_AGG(BINARY_CHECKSUM(*))FROMYour_Table_NameWITH (NOLOCK); That will return the same number each time its run as long as the table contents haven't changed. Unfortunately CHECKSUM does not work always properly to detect changes. It is only a primitive checksum and no CRC calculation. Therefore you can't use it to detect all changes, e. g. symmetrical changes result in the same CHECKSUM! Solution 2: 1.Run the following query. Before executing query replace DB_Name with your database name…

Merge and Merge join transformation in SSIS

MERGE TRANSFORMATION
Using Merge Transformation we can combine two sorted data-set into single data-set basically Merge Transformation used to combines rows from two sorted data flows into one sorted data flow. Following tasks you may perform using Merge Transformation: 1.Suppose we have a scenario like, we need to merge data from a database table and excel means we want to merge data from two different data sources. For such type of scenario, you can use Merge Transformation. 2.If we want to merge data from two same structured tables but exists two different servers. 3.Sometimes we get an error due to data in a row, after correcting errors in the data we can re-merge rows easily. See below explanations may help you to understand Merge Transformation: I do evaluate here, you already know about the data source, data conversion, data flow, task flow, control flow etc. Note:Before Merge transformation, we need to sort the data using Sort Transformation. After sorting data add data path to Merge…