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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

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