1 - like ‘%xx‘
 2     select * from tb1 where name like ‘%cn‘;
 3 - 使用函数
 4     select * from tb1 where reverse(name) = ‘wupeiqi‘;
 5 - or
 6     select * from tb1 where nid = 1 or email = ‘seven@live.com‘;
 7     特别的：当or条件中有未建立索引的列才失效，以下会走索引
 8             select * from tb1 where nid = 1 or name = ‘seven‘;
 9             select * from tb1 where nid = 1 or email = ‘seven@live.com‘ and name = ‘alex‘
10 - 类型不一致
11     如果列是字符串类型，传入条件是必须用引号引起来，不然...
12     select * from tb1 where name = 999;
13 - !=
14     select * from tb1 where name != ‘alex‘
15     特别的：如果是主键，则还是会走索引
16         select * from tb1 where nid != 123
17 - >
18     select * from tb1 where name > ‘alex‘
19     特别的：如果是主键或索引是整数类型，则还是会走索引
20         select * from tb1 where nid > 123
21         select * from tb1 where num > 123
22 - order by
23     select email from tb1 order by name desc;
24     当根据索引排序时候，选择的映射如果不是索引，则不走索引
25     特别的：如果对主键排序，则还是走索引：
26         select * from tb1 order by nid desc;
27  
28 - 组合索引最左前缀
29     如果组合索引为：(name,email)
30     name and email       -- 使用索引
31     name                 -- 使用索引
32     email                -- 不使用索引