5.35. PostgreSQL ALTER TABLE command

In PostgreSQL ALTER TABLE The command is used to add, modify, and delete a column that already exists in a table.

Besides, you can also use it. ALTER TABLE Command to add and remove constraints.

5.35.1. Grammar ¶

Use ALTER TABLE The syntax to add a column to an existing table is as follows:

ALTER TABLE table_name ADD column_name datatype;

On an existing table DROP COLUMN (delete column), the syntax is as follows:

ALTER TABLE table_name DROP COLUMN column_name;

To modify a column in a table DATA TYPE (data type), the syntax is as follows:

ALTER TABLE table_name ALTER COLUMN column_name TYPE datatype;

Add to a column in the table NOT NULL Constraints, the syntax is as follows:

ALTER TABLE table_name ALTER column_name datatype NOT NULL;

Give a column in a table ADD UNIQUE CONSTRAINT (add UNIQUE constraint), the syntax is as follows:

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

Give to the table ADD CHECK CONSTRAINT (add CHECK constraint), the syntax is as follows:

ALTER TABLE table_name
ADD CONSTRAINT MyUniqueConstraint CHECK (CONDITION);

Give the watch ADD PRIMARY KEY (add primary key), the syntax is as follows:

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

DROP CONSTRAINT (delete constraint), the syntax is as follows:

ALTER TABLE table_name
DROP CONSTRAINT MyUniqueConstraint;

If it’s MYSQL, the code looks like this:

ALTER TABLE table_name
DROP INDEX MyUniqueConstraint;

DROP PRIMARY KEY (delete the primary key), the syntax is as follows:

ALTER TABLE table_name
DROP CONSTRAINT MyPrimaryKey;

If it’s MYSQL, the code looks like this:

ALTER TABLE table_name
DROP PRIMARY KEY;

5.35.2. Example ¶

Create the COMPANY table ( 下载 COMPANY SQL 文件 ), the data are as follows:

runoobdb# select * from COMPANY;
 id | name  | age | address   | salary
----+-------+-----+-----------+--------
  1 | Paul  |  32 | California|  20000
  2 | Allen |  25 | Texas     |  15000
  3 | Teddy |  23 | Norway    |  20000
  4 | Mark  |  25 | Rich-Mond |  65000
  5 | David |  27 | Texas     |  85000
  6 | Kim   |  22 | South-Hall|  45000
  7 | James |  24 | Houston   |  10000
(7 rows)

The following example adds a new column to this table:

runoobdb=# ALTER TABLE COMPANY ADD GENDER char(1);

Now the watch looks like this:

 id | name  | age | address     | salary | gender
----+-------+-----+-------------+--------+--------
| Paul  |  32 | California  |  20000 |
| Allen |  25 | Texas       |  15000 |
| Teddy |  23 | Norway      |  20000 |
| Mark  |  25 | Rich-Mond   |  65000 |
| David |  27 | Texas       |  85000 |
| Kim   |  22 | South-Hall  |  45000 |
| James |  24 | Houston     |  10000 |
(7 rows)

Delete the following example GENDER Column:

runoobdb=# ALTER TABLE COMPANY DROP GENDER;

The results are as follows:

id | name  | age | address   | salary
----+-------+-----+-----------+--------
| Paul  |  32 | California|  20000
| Allen |  25 | Texas     |  15000
| Teddy |  23 | Norway    |  20000
| Mark  |  25 | Rich-Mond |  65000
| David |  27 | Texas     |  85000
| Kim   |  22 | South-Hall|  45000
| James |  24 | Houston   |  10000