Relational Query Languages | Data Definition language(DDL)

DDL(Data Definition Language)

The Data Definition Language (DDL) manages table and index structure.
COMMAND USED- CREATE, ALTER, DROP,RENAME
CREATE creates an object (a table) in the database.
ALTER modifies the structure of an existing object in various ways, for    example, adding a column to an existing table or a constraint.
DROP deletes an object in the database, usually irretrievably, i.e., it cannot be rolled back.
RENAME rename scheme object

1. CREATE COMMAND-This command is used when we want to create a new table.

syntax: create table <tname>  (col.1 data type (size) constraint <constraint name> not null , col.2 data type (size));

E.g.: create table mydoc1 (username varchar (20) constraint name_mn not null, password number(10));

2. ALTER COMMAND- This command is used when we want to delete a particular column from a existing table.

We use three clauses with alter command:

1. ADD

2. MODIFY

3. DROP

·   ADD: This command is used when we want to create a new column.

syntax: alter table tname add (col. name data type (size));

E.g.: alter table student add (Roll no. number(100));

·   MODIFY: This command is used when we want to modify the table.

syntax: alter table tname modify (col. name data type(size));

E.g.: alter table emp modify (Branch  varchar(20));

3. DROP COMMAND- This command is used when we want to drop a table.

syntax: alter table tname drop column (col. name);

E.g.: alter table emp drop column salary; 

4.RENAME COMMAND-Rename scheme object

SYNTAX- alter table <tname> RENAME COLUMN (old column name to new column name);

E.G.- alter table emp RENAME COLUMN (name to empname);