添加新的 MySQL 表列并创建索引?
mysqlmysqli database
要添加新的 MySQL 表列和索引，可以使用 ALTER TABLE 命令。
语法如下
ALTER TABLE yourTableName ADD COLUMN yourColumnName dataType, ADD INDEX(yourColumnName );
为了理解上述语法，让我们创建一个表。创建表的查询如下
mysql> create table AddColumnAndIndexDemo -> ( -> Id int NOT NULL AUTO_INCREMENT PRIMARY KEY, -> Name varchar(100), -> Address varchar(200) -> ); Query OK, 0 rows affected (0.81 sec)
现在可以查看表的描述。查询如下 −
mysql> desc AddColumnAndIndexDemo;
下面是输出结果
+---------+--------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +---------+--------------+------+-----+---------+----------------+ | Id | int(11) | NO | PRI | NULL | auto_increment | | Name | varchar(100) | YES | | NULL | | | Address | varchar(200) | YES | | NULL | | +---------+--------------+------+-----+---------+----------------+ 3 rows in set (0.03 sec)
以下是添加新 MySQL 表列和索引的查询
mysql> alter table AddColumnAndIndexDemo add column Age int, add index(Age); Query OK, 0 rows affected (1.81 sec) Records: 0 Duplicates: 0 Warnings: 0
再次检查表描述。查询如下 −
mysql> desc AddColumnAndIndexDemo;
下面是输出结果
+---------+--------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +---------+--------------+------+-----+---------+----------------+ | Id | int(11) | NO | PRI | NULL | auto_increment | | Name | varchar(100) | YES | | NULL | | | Address | varchar(200) | YES | | NULL | | | Age | int(11) | YES | MUL | NULL | | +---------+--------------+------+-----+---------+----------------+ 4 rows in set (0.00 sec)
使用 SHOW 命令检查表中的索引。查询如下 −
mysql> show index from AddColumnAndIndexDemo;
下面是输出结果
+-----------------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | +-----------------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+ | addcolumnandindexdemo | 0 | PRIMARY | 1 | Id | A | 0 | NULL | NULL | | BTREE | | | YES | | addcolumnandindexdemo | 1 | Age | 1 | Age | A | 0 | NULL | NULL | YES | BTREE | | | YES | +-----------------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+ 2 rows in set (0.16 sec)
