复制# 建表时指定索引  CREATETABLE `t_index` (    `increment_id` int(11) NOTNULL AUTO_INCREMENT COMMENT 自增主键,创建索引    `col1` int(11) NOTNULL,    `col2` varchar(20) NOTNULL,    `col3` varchar(50) NOTNULL,    `col4` int(11) NOTNULL,   `col5` varchar(50) NOTNULL,    PRIMARYKEY (`increment_id`),    UNIQUEKEY `uk_col1` (`col1`),    KEY `idx_col2` (`col2`)  ) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COMMENT=测试索引;  # 创建索引(两种方法)  # 普通索引  altertable `t_index` addindex idx_col3 (col3);   createindex idx_col3 on t_index(col3);  # 唯一索引  altertable `t_index` adduniqueindex uk_col4 (col4);  createuniqueindex uk_col4 on t_index(col4);  # 联合索引  altertable `t_index` addindex idx_col3_col4 (col3,col4);  createindex idx_col3_col4 on t_index(col3,col4);  # 前缀索引  altertable `t_index` addindex idx_col5 (col5(20));   createindex idx_col5 on t_index(col5(20));  # 查看表索引  mysql> show indexfrom t_index;  +---------+------------+----------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table   | Non_unique | Key_name | Seq_in_index | Column_name  | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |  +---------+------------+----------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | t_index |          0 | PRIMARY  |            1 | increment_id | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |  | t_index |          0 | uk_col1  |            1 | col1         | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |  | t_index |          1 | idx_col2 |            1 | col2         | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |  | t_index |          1 | idx_col3 |            1 | col3         | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |  +---------+------------+----------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30.31.32.33.34.35.36.37.