where条件判断

字符匹配，条件为字段A大于字段B
```asp
select username,uid,gid from db3.user where uid > gid;
```
字符匹配，条件为字段A等于字段B
```asp
select username,uid,gid from db3.user where uid = gid;
```
字符匹配，条件为字段A不等于（！取反）字段B
```asp
select username,uid,gid from db3.user where uid != gid;
```
范围内匹配，in包含内容匹配
```asp
select username from db3.user where username in ("mysql","nginx");
```
范围内匹配，not in不包含内容匹配
```asp
select username from db3.user where username not in ("mysql","nginx");
```
范围内匹配，某个区间的内容匹配
```asp
select id,username from db3.user where id between 10 and 20;
```
范围内匹配，去掉重复值
```asp
select distinct shell from db3.user;
```
匹配空null
```asp
select username from db3.user where comment is null;
```
匹配非空not null
```asp
select username from db3.user where comment is not null;
```
逻辑匹配，1个条件成立匹配
```asp
select username,uid from db3.user where username="root" or uid=1;
```
逻辑匹配，2个条件同时成立匹配
```asp
select username,uid from db3.user where username="root" and uid=0;
```
模糊匹配，like匹配_代表1个字符，满足1个字符匹配
```asp
select username from db3.user where username like '_';
```
模糊匹配，%%内包含字符即匹配
```asp
select username from db3.user where username like '%a%';
```
模糊匹配，以a开头的即匹配
```asp
select username from db3.user where username like 'a%';
```
模糊匹配，至少4个字符即匹配
```asp
select username from db3.user where username like '__%__';
```
正则匹配，元字符^开头的匹配
```asp
select username from db3.user where username regexp '^a|^d';
```
正则匹配，元字符^开头[]和|一个效果
```asp
select username from db3.user where username regexp '^[ad]';
```
正则匹配，包含范围内数字即匹配
```asp
select username from db3.user where username regexp '[0-9]';
```
匹配结果并排序，asc对匹配出来的结果做升序（asc默认可以不加）
```asp
select username,uid from user where uid <=1000 order by uid asc;
```
匹配结果并排序，desc对匹配出来的结果做降序
```asp
select username,uid from user where uid <=1000 order by uid desc;
```
匹配结果并分组，group by对匹配出来的结果做分组
```asp
select shell from user where uid <=100 group by shell;
```
匹配结果并再匹配一次，having对匹配出来的结果在按照条件匹配一次
```asp
select * from user where username is not null having uid <=30;
```
限制查询结果显示行数，linit+数字多少就显示开头的多少行
```asp
select * from user limit 2;
```
限制查询结果显示行数，linit+行数+步长（第2行开始输出5行）
```asp
select * from user limit 2,5;
```