SQL - ALTER TABLE Command

The SQL ALTER TABLE command is used to add, delete, or modify columns in an existing table.
You would also use ALTER TABLE command to add and drop various constraints on a an existing table.

Syntax:

The basic syntax of ALTER TABLE to add a new column in an existing table is as follows:

ALTER TABLE table_name ADD column_name datatype;

The basic syntax of ALTER TABLE to DROP COLUMN in an existing table is as follows:

ALTER TABLE table_name DROP COLUMN column_name;

The basic syntax of ALTER TABLE to change the DATA TYPE of a column in a table is as follows:

ALTER TABLE table_name MODIFY COLUMN column_name datatype;

The basic syntax of ALTER TABLE to add a NOT NULL constraint to a column in a table is as follows:

ALTER TABLE table_name MODIFY column_name datatype NOT NULL;

The basic syntax of ALTER TABLE to ADD UNIQUE CONSTRAINT to a table is as follows:

ALTER TABLE table_name 
ADD CONSTRAINT MyUniqueConstraint UNIQUE(column1, column2...);

The basic syntax of ALTER TABLE to ADD CHECK CONSTRAINT to a table is as follows:

ALTER TABLE table_name 
ADD CONSTRAINT MyUniqueConstraint CHECK (CONDITION);

The basic syntax of ALTER TABLE to ADD PRIMARY KEY constraint to a table is as follows:

ALTER TABLE table_name 
ADD CONSTRAINT MyPrimaryKey PRIMARY KEY (column1, column2...);

The basic syntax of ALTER TABLE to DROP CONSTRAINT from a table is as follows:

ALTER TABLE table_name 
DROP CONSTRAINT MyUniqueConstraint;

If you're using MySQL, the code is as follows:

ALTER TABLE table_name 
DROP INDEX MyUniqueConstraint;

The basic syntax of ALTER TABLE to DROP PRIMARY KEY constraint from a table is as follows:

ALTER TABLE table_name 
DROP CONSTRAINT MyPrimaryKey;

If you're using MySQL, the code is as follows:

ALTER TABLE table_name 
DROP PRIMARY KEY;

Example:

Consider CUSTOMERS table is having following records:

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Following is the example to ADD a new column in an existing table:

ALTER TABLE CUSTOMERS ADD SEX char(1);

Now CUSTOMERS table is changed and following would be output from SELECT statement:

+----+---------+-----+-----------+----------+------+
| ID | NAME    | AGE | ADDRESS   | SALARY   | SEX  |
+----+---------+-----+-----------+----------+------+
|  1 | Ramesh  |  32 | Ahmedabad |  2000.00 | NULL |
|  2 | Ramesh  |  25 | Delhi     |  1500.00 | NULL |
|  3 | kaushik |  23 | Kota      |  2000.00 | NULL |
|  4 | kaushik |  25 | Mumbai    |  6500.00 | NULL |
|  5 | Hardik  |  27 | Bhopal    |  8500.00 | NULL |
|  6 | Komal   |  22 | MP        |  4500.00 | NULL |
|  7 | Muffy   |  24 | Indore    | 10000.00 | NULL |
+----+---------+-----+-----------+----------+------+

Following is the example to DROP sex column from existing table:

ALTER TABLE CUSTOMERS DROP SEX;

Now CUSTOMERS table is changed and following would be output from SELECT statement:

+----+---------+-----+-----------+----------+
| ID | NAME    | AGE | ADDRESS   | SALARY   |
+----+---------+-----+-----------+----------+
|  1 | Ramesh  |  32 | Ahmedabad |  2000.00 |
|  2 | Ramesh  |  25 | Delhi     |  1500.00 |
|  3 | kaushik |  23 | Kota      |  2000.00 |
|  4 | kaushik |  25 | Mumbai    |  6500.00 |
|  5 | Hardik  |  27 | Bhopal    |  8500.00 |
|  6 | Komal   |  22 | MP        |  4500.00 |
|  7 | Muffy   |  24 | Indore    | 10000.00 |
+----+---------+-----+-----------+----------+