Alter table command
We can change the definition of a table even after creating it. For this, ALTER TABLE command is used. This command can be used to add columns to the table, change their sizes, change their datatypes or add or delete constraints.
The syntax for ALTER TABLE command is as mentioned below:
ALTER TABLE <TABLE NAME > [options]ADD
Add keyword is used to add columns or constraints to the table. For example to a column to an existing table we use following command
ALTER TABLE TABLE NAME ADD COLUMN NAME DATA TYPE(SIZE);Example.
ALTER TABLE EMPLOYEE ADD PHONE CHAR(10);MODIFY
Modify option can change the data type, column width and constraints of an existing column. For example to change the column width and data type of phone of employee table, we can use the following statement.
ALTER TABLE EMPLOYEE MODIFY PHONE NUMBER(10)DROP
To remove the constraints imposed on the table, drop option can be used. For example the following statement will remove the constraint check_code
ALTER TABLE EMPLOYEE DROP CONSTRAINT CHECK_CODE
