Data Definition Language:-

These commands are used for creating ,modifying and dropping the structure of Database objects.

This language contains five commands

1.     Create

2.     Alter

3.     Sp_Rename

4.     Truncate

5.     Drop

      
1.Create: 

• This command is used to create the database objects within the database
• SY: create table <table name>

         (col 1 data type (size),

         col2 data type (size),

                     :

                     :

                     :

                     :

         coln data type (size));

Ex: create table Employee(EmpId int,EName varchar(20))  --Creating A table

2. ALTER:

• This command is used to modify the structure of a table using this command, we can perform four different operations
• Using this command we can increase (or) decrease the size of the data type & also we can change the data type from old data type to new data type
• We can add a new column to the existing table
• We can change the column name from old column name to new column name
• We can remove the column from the existing table

This command contains 4 sub commands

1.     Alter- Alter column

2.     Alter- Add

3.     Sp_Rename

4.     Alter- Drop

SP_help: This command is used to see the structure of table

SY: sp_help <table name>

Ex: sp_help Employee

Syntax for Alter-Alter Column

Sy: Alter Table <Table Name> Alter Column Column Name Data Type (Size)

Ex: alter table Employee Alter column EName varchar(22)—-Altering the coloumn

  

i]Alter-Add: This command is used to add a new column to the existing table

Sy: ALTER TABLE <TABLE NAME> ADD COLUMNNAME DATA TYPE(size);

Ex:- alter table Employee add  Salary money –-ading the coloum to the existing table

ii]Alter-Drop:

Alter Table <Table Name> Drop Column <Column Name>;

alter table employee drop column salary—to delete a perticular coloumn

iii]Sp_Rename:

SY: Sp_Rename ‘Tablename.Oldcolumn’,’New Column Name’,’Column’;

EX: sp_rename 'Employee.EName','Name','column' –changing the column Name of the table.

3. Sp_Rename:

This command is used to change the table name from old table name to new table name

SY: sp_rename ‘old table name’,’New table name’

EX: sp_rename empdetails,Employee—changing the Name of the table

4. Truncate:

This command is used for to delete all the records from existing table permanently

Sy: Truncate Table <Table Name>

EX: truncate table employee –-to delete the records permanently from the database

5. Drop:

This command is used to remove the table permanently from the database

Sy: Drop Table <Table Name>

Ex: drop table employee –to drop the table permanently from the database