sql索引分析-插入了 a、b、c、d 四个字段作为索引，只要带上了a，那么任何排列的组合，都可以走索引。

sql索引分析

1、如果创建了一个索引
 

一

ALTER TABLE `table_A` ADD KEY `nid_sn_key`(`a`,`b`,`c`,`d`) USING BTREE;

第一种情况：

explain SELECT * FROM `table_A` WHERE `a` = "xxx"；

explain SELECT * FROM `table_A` WHERE `a` != "xxx"；

会走索引

第二种情况：

explain SELECT * FROM `table_A` WHERE `b` = "xxx"；

不走索引

等等，如果单独查询c， 或者d，都不会走索引；

第三种情况：

explain SELECT * FROM `table_A` WHERE `a` = "xxx", and `b` = "xxx"；

explain SELECT * FROM `table_A` WHERE `a` != "xxx", and `b` = "xxx"；

explain SELECT * FROM `table_A` WHERE `a` = "xxx", and `b` != "xxx"；

explain SELECT * FROM `table_A` WHERE `a` != "xxx", and `b` != "xxx"；

explain SELECT * FROM `table_A` WHERE `b` = "xxx" and `a` = "xxx",

会走索引

第四种情况：

explain SELECT * FROM `table_A` WHERE `a` = "xxx" and `b` = "xxx" and `c` ="xxx";

explain SELECT * FROM `table_A` WHERE `b` = "xxx" and `a` = "xxx" and `c` ="xxx";

explain SELECT * FROM `table_A` WHERE `b` = "xxx" and `c` ="xxx" and `a` = "xxx" ; 

会走索引

总结 插入了 a、b、c、d 四个字段作为索引，只要带上了a，那么任何排列的组合，都可以走索引。

二