建立表：
CREATE TABLE `ygzt_test` (
`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`a` int(11) NOT NULL,
`b` int(11) NOT NULL,
`c` int(11) NOT NULL,
`d` int(11) NOT NULL,
PRIMARY KEY (`id`),
KEY `a` (`a`,`b`,`c`,`d`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci COMMENT='测试';
一、实验一，无order by
首先加联合索引a,b,c,d
explain select * from ygzt_test where a=1 and b=2 and c=3 and d=4
修改sql：
explain select * from ygzt_test where a>1 and b=2 and c=3 and d=4
type已经由ref降为index
修改索引b,c,d,a
explain select * from ygzt_test where a>1 and b=2 and c=3 and d=4
done
二、实验二，order by
建立索引a，b
explain select * from ygzt_test where a>0 order by b
可以看到，a>0使用了索引，order by b 未使用
修改索引为b，a
explain select * from ygzt_test where a>0 order by b
where 与 order by 都无索引
想起此前order by＋select ＊的问题
这个问题单独拿出来实践下：
修改索引为b
explain select * from ygzt_test order by b
缺点：未使用索引
explain select b from ygzt_test order by b
缺点：只能返回b
explain select b,a from ygzt_test order by b
缺点：未使用索引
explain select * from ygzt_test FORCE INDEX (b) order by b
缺点：type也就是index级别
索引b，a
explain select b,a from ygzt_test order by b
缺点：与强制索引同，只能达到index级别，还加大了索引建立的复杂度
结论：使用强制索引
结论：
1.单列索引中——,>=,between,like(右边模糊)适用索引
2.索引中有范围的，有序性失效，解决方案以实际为准
