2020-11-08：在Mysql中，三个字段A、B、C的联合索引，查询条件是B、A、C，会用到索引吗？

福哥答案2020-11-08：

会走索引，原因是mysql优化器会把BAC优化成ABC。

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CREATE TABLE `t_testabc2` (   `id` int(11) NOT NULL AUTO_INCREMENT,   `A` varchar(255) NOT NULL,   `B` varchar(255) NOT NULL,   `C` varchar(255) NOT NULL,   `D` varchar(255) NOT NULL DEFAULT '''''',   PRIMARY KEY (`id`),   KEY `abc2` (`A`,`B`,`C`) USING BTREE ) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=latin1;   EXPLAIN SELECT * FROM t_testabc2 WHERE B='a2' AND A='a2' AND C='a2'    --走索引   EXPLAIN SELECT * FROM t_testabc2 WHERE  B >='a1' AND B<='a2' AND A >='a1' AND A<='a2' AND C >='a1' AND C<='a2'    --走索引   EXPLAIN SELECT * FROM t_testabc2 WHERE  B BETWEEN 'a1' AND 'a2' AND A BETWEEN 'a1' AND 'a2' AND C BETWEEN 'a1' AND 'a2'    --走索引   EXPLAIN SELECT * FROM t_testabc2 WHERE B='a1' AND A LIKE 'a%' AND C='a1'    --不走索引