sql alert table 修改表结构的使用方法

ALTER TABLE 语句用于在已有的表中添加、修改或删除列

ADD [COLUMN] column name (column definitions) [FIRST or AFTER column_name]
 ADD INDEX [index_name] (column_list)
 ADD PRIMARY KEY (column_list)
 ADD UNIQUE [index_name] (column_list)
 ALTER [COLUMN] column_name {SET DEFAULT default_value or DROP DEFAULT}
 CHANGE [COLUMN] old_col_name create_definition
 DROP [COLUMN] col_name
 DROP PRIMARY KEY
 DROP INDEX index_name
 MODIFY [COLUMN] create_definition
 RENAME [AS] new_tbl_name

The IGNORE keyword causes rows with duplicate values in unique keys to be deleted;
 otherwise, nothing happens.

ALTER TABLE employee ADD COLUMN Account_Number INT
 ALTER TABLE employee ADD INDEX (ID)
 ALTER TABLE employee ADD PRIMARY KEY (ID)
 ALTER TABLE employee ADD UNIQUE (ID)
 ALTER TABLE employee CHANGE ID salary INT
 ALTER TABLE employee DROP Customer_ID
 ALTER TABLE employee DROP PRIMARY KEY

ALTER TABLE employee DROP INDEX Customer_ID
 ALTER TABLE employee MODIFY First_Name varchar(100)
 ALTER TABLE employee RENAME Customer

给表增加一个主键

mysql教程>
 mysql>
 mysql> CREATE TABLE myTable(
     ->    ID SMALLINT
     -> );
 Query OK, 0 rows affected (0.02 sec)

mysql>
 mysql> desc myTable;
 +-------+-------------+------+-----+---------+-------+
 | Field | Type        | Null | Key | Default | Extra |
 +-------+-------------+------+-----+---------+-------+
 | ID    | smallint(6) | YES  |     | NULL    |       |
 +-------+-------------+------+-----+---------+-------+
 1 row in set (0.00 sec)

mysql>
 mysql> ALTER TABLE myTable
     ->    ADD COLUMN Quantity SMALLINT UNSIGNED NOT NULL,
     ->    MODIFY ID SMALLINT UNSIGNED NOT NULL,
     ->    ADD PRIMARY KEY (ID);
 Query OK, 0 rows affected (0.05 sec)
 Records: 0  Duplicates: 0  Warnings: 0

mysql>
 mysql> desc myTable;
 +----------+----------------------+------+-----+---------+-------+
 | Field    | Type                 | Null | Key | Default | Extra |
 +----------+----------------------+------+-----+---------+-------+
 | ID       | smallint(5) unsigned | NO   | PRI |         |       |
 | Quantity | smallint(5) unsigned | NO   |     |         |       |
 +----------+----------------------+------+-----+---------+-------+
 2 rows in set (0.02 sec)

mysql>
 mysql> drop table myTable;
 Query OK, 0 rows affected (0.00 sec)

mysql>

修改表列名

mysql>
 mysql> ALTER TABLE myTable
     ->    DROP COLUMN Quantity,
     ->    DROP PRIMARY KEY;
 Query OK, 0 rows affected (0.05 sec)
 Records: 0  Duplicates: 0  Warnings: 0