PostgreSQL修改表（ALTER TABLE语句）及添加和删除各种约束

PostgreSQL ALTER TABLE命令用于添加，删除或修改现有表中的列。您还可以使用ALTER TABLE命令在现有表上添加和删除各种约束。文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4955.html

语法：
使用ALTER TABLE语句在现有表中添加新列的基本语法如下：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4955.html

ALTER TABLE table_name ADD column_name datatype;

现有表中ALTER TABLE到DROP COLUMN(删除某个字段)的基本语法如下：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4955.html

ALTER TABLE table_name DROP COLUMN column_name;

ALTER TABLE更改表中列的DATA TYPE(修改字段类型)的基本语法如下：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4955.html

ALTER TABLE table_name ALTER COLUMN column_name TYPE datatype;

ALTER TABLE向表中的列添加NOT NULL约束的基本语法如下：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4955.html

ALTER TABLE table_name MODIFY column_name datatype NOT NULL;

ALTER TABLE添加唯一约束ADD UNIQUE CONSTRAINT到表中的基本语法如下：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4955.html

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

ALTER TABLE将“检查约束”添加到表中的基本语法如下所示：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4955.html

ALTER TABLE table_name
ADD CONSTRAINT MyUniqueConstraint CHECK (CONDITION);

ALTER TABLE添加主键ADD PRIMARY KEY约束的基本语法如下：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4955.html

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

使用ALTER TABLE从表中删除约束(DROP CONSTRAINT)的基本语法如下：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4955.html

ALTER TABLE table_name
DROP CONSTRAINT MyUniqueConstraint;

使用ALTER TABLE从表中删除主键约束(DROP PRIMARY KEY)约束的基本语法如下：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4955.html

ALTER TABLE table_name
DROP CONSTRAINT MyPrimaryKey;

示例：

考虑一个表 - COMPANY 有以下记录：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4955.html

 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

以下是向现有的COMPANY表中添加新列的示例：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4955.html

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

现在，COMPANY表被更改，以下使用SELECT语句输出：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4955.html

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)

以下是现有表中删除 gender列的示例：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4955.html

yiibai_db=# ALTER TABLE COMPANY DROP GENDER;

现在，COMPANY表被更改，以下将从SELECT语句输出：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/sjk/4955.html

 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