SQL SERVER : ALTER, DELETE,DROP

SQL SERVER-ALTER, DELETE, DROP

We will use below table for demo purpose

CREATE TABLE Emp_Test

 (EmpID int,

 Empname varchar(10))

ALTER: ALTER command is used to do below things

•    Add column/constrains
•    Modify column
•    Drop column/Constraint

Add Column syntax:

ALTER TABLE <<Table>>

ADD Column_name datatype [size]

Example:

ALTER TABLE Emp_test

ADD Dateofbirth date

We can also add constraints to the existing table. We will discuss about constrains in future sessions. For now we will learn how to add constraints 

Add Constraint Syntax:

ALTER TABLE <<tablename>>

ADD Constraint  Constraint_name ConstraintType (Columns…)

Example:

ALTER TABLE Emp_test

ADD constraint EMP_Unique UNIQUE(EMPID) – Here Unique means EMPID column data should not be repeated

Modify column:

We can also modify the existing column data type and its size if table is empty and it table has data and then we are trying to modify the existing column data type then if the new data type is compatible to the existing data in the table then it will allow other it will through error. E.g. existing column was created using char(10) then if we are trying to modify to Varchar(20)  then it won’t throw any error. Compatibility data types are always we can change. 

Syntax:

ALTER TABLE <<Table_name>> 

ALTER Column Col_name datatype

Example:

ALTER TABLE Emp_test

ALTER Column Empname char(12)

Drop column/Constraint: We can  drop existing column/constraint using ALTER command 

Syntax: 

ALTER TABLE <<table_name>>

DROP <<Column or Constraint>> <<Column_name>>

Example:

Drop Empname column

ALTER TABLE Emp_test

DROP Column Empname  

Drop unique constraint (EMP_Unique)

ALTER TABLE Emp_test

DROP CONSTRAINT EMP_Unique

DELETE:  DELETE Command is used to delete table rows. We can delete all the rows from a table or we can also delete specific rows by using WHERE condition. 

Syntax:

DELETE FROM <<table name>> 

<<WHERE some conditions>>

Example:

DELETE FROM Emp_test--Delete all the rows

DELETE FROM Emp_test WHERE EmpID=10--Delete rows which are having empid=10

DELETE FROM Emp_test WHERE EmpID<10 --Delete rows which are having empid<10

DELETE FROM Emp_test WHERE EmpID<>10 ----Delete rows which are having empid<>10 

In SQL server we can also write DELETE statement like below

DELETE emp_test

TRUNCATE: TRUNCATE Command is also used to delete the data from table

Syntax:

TRUNCATE TABLE <<Tablename>>

Example:

TRUNCATE TABLE emp_test

Here we should get one question like 

What is the difference between DELETE and TRUNCATE?

 Both are deleting rows. Yes both are deleting table data however there are some differences are there while deleting the data.

      1.     For DELETE We can use WHERE condition whereas TRUNCATE We can’t give 

      2.     DELETE Command will maintain log for each deleted row whereas TRUNCATE will not maintain any log

      3.     We can use some joins to delete the data where as in TRUNCATE we can’t use any joins

      4.     DELETE Will not reset IDENTITY [We will discuss in future sessions about IDENTITY] Whereas TRUNCATE Will      reset IDENTITY

      5.     We can Roll back the data when we use DELETE whereas TRUNCATE we can’t rollback

Overall, if we want to delete all the row then it’s always go with TRUNCATE Command

DROP: DROP Command is used to drop the table

Syntax:

DROP TABLE <<tablename>>

Example:

DROP TABLE emp_test