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 ----+-------+-----+-------------+--------+-------- 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) 

Delete the following example GENDER Column:

runoobdb=# ALTER TABLE COMPANY DROP GENDER; 

The results are as follows:

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 