The SQL ALTER TABLE command is used to modify the definition (structure) of a table by modifying the definition of its columns. The ALTER command is used to perform the following functions.
1) Add, drop, modify table columns
2) Add and drop constraints
3) Enable and Disable constraints
Syntax to add a column
ALTER TABLE table_name ADD column_name datatype;
For Example: To add a column "experience" to the employee table, the query would be like
ALTER TABLE employee ADD experience number(3);
Syntax to drop a column
ALTER TABLE table_name DROP column_name;
For Example: To drop the column "location" from the employee table, the query would be like
ALTER TABLE employee DROP location;
Syntax to modify a column
ALTER TABLE table_name MODIFY column_name datatype;
For Example: To modify the column salary in the employee table, the query would be like
ALTER TABLE employee MODIFY salary number(15,2);
The SQL RENAME command is used to change the name of the table or a database object.
If you change the object's name any reference to the old name will be affected. You have to manually change the old name to the new name in every reference.
Syntax to rename a table
RENAME old_table_name To new_table_name;