8.like以通配符开头（'$abc...'）mysql索引失效会变成全表扫描操作
EXPLAIN SELECT * FROM employees WHERE name like '%Lei';
EXPLAIN SELECT * FROM employees WHERE name like 'Lei%';
问题：解决like'%字符串%'索引不被使用的方法？ a）使用覆盖索引，查询字段必须是建立覆盖索引字段
EXPLAIN SELECT name,age,position FROM employees WHERE name like '%Lei%';
b）当覆盖索引指向的字段是varchar(380)及380以上的字段时，覆盖索引会失效！
9.强制类型转换导致索引失效
强制类型转换导致索引失效, 比如：字符串不加单引号索引失效
EXPLAIN SELECT * FROM employees WHERE name = '1000';
EXPLAIN SELECT * FROM employees WHERE name = 1000;
10.少用or,用它连接时很多情况下索引会失效
EXPLAIN SELECT * FROM employees WHERE name = 'LiLei' or name = 'HanMeimei';
小总结
like KK%相当于=常量，%KK和%KK% 相当于范围
总结 1
总结 2
