ALTER TABLE

This command is used to Add/Change/Modify/Drop existing structure of the table.

ADD Column
Enable/Disable Constraints
Change Column
Modify Column
Drop Column

ADD Column :- When a new column is to be added to the table structure without constraints.

Syntax :-

ALTER TABLE table_name
ADD COLUMN column_name datatype (size);

Example:-

ALTER TABLE my_tab
ADD COLUMN stu_id integer (5);

Change Column :-This is used to change name and data type of an existing column without constraints.

Syntax:-

ALTER TABLE table_name
CHANGE COLUMN old_column_name new_column_name new_data_type (size);

Example:-

ALTER TABLE my_tab
CHANGE COLUMN name student varchar (5);

Modify Column :- This is used to modify size of the data type or the data type itself of an existing column without changing column name.

Syntax:-

ALTER TABLE table_name
MODIFY COLUMN column_name datatype (size);

Example:-

ALTER TABLE  my_tab
MODIFY COLUMN roll integer (10);

DROP COLUMN :- When a column in a table need to delete

Syntax :- 

ALTER TABLE table_name
DROP COLUMN column_name;

Example:-

ALTER TABLE my_tab
DROP COLUMN roll;

When removing constraints from a column

Syntax:-

ALTER TABLE table_name
DROP constraints_name column_name;

Example:-

ALTER TABLE my_tab
DROP UNIQUE KEY (roll);