鍍金池/ 教程/ 數(shù)據(jù)庫(kù)/ SQL ALTER TABLE(修改表)
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SQL ALTER TABLE(修改表)

SQL ALTER TABLE命令用于添加,刪除或修改列在一個(gè)現(xiàn)有的表。

也可以使用ALTER TABLE命令可在現(xiàn)有的表中添加和刪除各種約束。

語(yǔ)法

ALTER TABLE基本語(yǔ)法來(lái)添加新的列到現(xiàn)有表如下:

ALTER TABLE table_name ADD column_name datatype;

ALTER TABLE用于DROP COLUMN在現(xiàn)有表的基本語(yǔ)法如下:

ALTER TABLE table_name DROP COLUMN column_name;

ALTER TABLE的基本語(yǔ)法更改列的數(shù)據(jù)類型(DATA TYPE)在表中如下:

ALTER TABLE table_name MODIFY COLUMN column_name datatype;

ALTER TABLE添加NOT NULL約束到一個(gè)表的列的基本語(yǔ)法如下:

ALTER TABLE table_name MODIFY column_name datatype NOT NULL;

ALTER TABLE添加唯一約束到表的基本語(yǔ)法如下:

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

ALTER TABLE添加CHECK約束到表的基本語(yǔ)法如下:

ALTER TABLE table_name 
ADD CONSTRAINT MyUniqueConstraint CHECK (CONDITION);

ALTER TABLE添加PRIMARY KEY約束到表的基本語(yǔ)法如下:

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

ALTER TABLE從表中刪除約束的基本語(yǔ)法如下:

ALTER TABLE table_name 
DROP CONSTRAINT MyUniqueConstraint;

如果使用MySQL,參考代碼如下:

ALTER TABLE table_name 
DROP INDEX MyUniqueConstraint;

ALTER TABLE從表中刪除主鍵約束的基本語(yǔ)法如下:

ALTER TABLE table_name 
DROP CONSTRAINT MyPrimaryKey;

如果使用MySQL,代碼如下:

ALTER TABLE table_name 
DROP PRIMARY KEY;

例子:

考慮CUSTOMERS表具有以下記錄:

+----+----------+-----+-----------+----------+
| 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 |
+----+----------+-----+-----------+----------+

以下是添加一個(gè)新列到現(xiàn)有的表的示例:

ALTER TABLE CUSTOMERS ADD SEX char(1);

現(xiàn)在對(duì)CUSTOMERS表作出改變,下面是來(lái)自SELECT語(yǔ)句的輸出:

+----+---------+-----+-----------+----------+------+
| 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 |
+----+---------+-----+-----------+----------+------+