表记录的基本操作
查看 select
select 字段名列表 from 数据库名.表名；
select 字段名列表 from 数据库名.表名 where 条件表达式；
*
字段名1，字段名2，字段名N
条件的表示方式：
使用where引导，适用于更新、删除、查询等场合，符合条件的记录才被操作
1.数值比较
等于： =
不等于： !=
大于、大于且等于： > >=
小于、小于且等于 < <=
在..与..之间：between .. and ..
2. 逻辑比较 （查询条件是2个或2个以上时使用）
and 并且 多个条件同时成立
or 或 多个条件只要有一个条件成立就可以
！ 非 （ 取反） = ！=
select * from usertab where username="root" and uid=20;
select * from usertab where username!="root" and uid!=20;
3.范围内查找
(1)in 在....里
select username,uid,shell from usertab where uid in (500,501,10);
select username,uid,shell from usertab where username in ("root","bin","sync");
select username,uid,shell from usertab where uid=500 or uid=501 or uid=10;
(2)not in 不在....里
select username,uid,shell from usertab where uid not in (500,501,10);
(3)between .... and .... 同时匹配两个条件
select * from usertab where uid>=5 and uid<=10;
select * from usertab where uid between 5 and 10;
select username from usertab where username between "a" and "f";
（4）is null 空
（5） is not null 非空
select * from usertab where shell is not null;
4.排序
order by
升序排序:asc ( 默认 )
降序排序:desc
order by 字段名 desc
样例：
把uid号最大的前5个用户信息输出
select * from usertab order by uid desc limit 5;
select * from usertab limit 1;
5.设置显示记录的条目数
limit
limit N,M
limit M
N 从查询结果的第几条记录开始显示，不指定N的值，默认从查询结果的第一条 记录开始显示，第一条记录的编号是0
M 共显示几条记录
select * from usertab limit 2;
select * from usertab limit 5,6;
select * from usertab order by uid desc limit 5;
6.分组
group by
group by 字段名
select shell from usertab group by shell;
select shell from usertab where uid>=500 group by shell;
7.不显示字段的重复值
DISTINCT
select distinct shell from usertab；
8.模糊查询
like
% 匹配零个到多个字符
_ 匹配任意一个字符
select * from usertab where username like "r%";
select * from usertab where username like "___";
select * from usertab where username like "r___";
select * from usertab where username like "_r_";
select * from usertab where username like "r%r";
select * from usertab where username like "_%_";
9.正则表达式
regexp
WHERE 字段名 REGEXP ‘正则表达式‘
^ 匹配开头
$ 匹配结尾
. 匹配任意当个字符
[0-9] 匹配所有数字
[0123456789]
[a-z]
select username from usertab where username regexp ‘o‘;
select username from usertab where username regexp ‘...‘;
select username from usertab where username regexp ‘^....$‘;
select username from usertab where username regexp ‘^a...$‘;
select username from usertab where username regexp ‘^s‘;
select username from usertab where username regexp ‘[0-9]‘;
10.四则运算
求年龄：
alter table usertab add syear year default "1987" after username;
select username,2014-syear as age from usertab;
select username,uid+gid as zcj from usertab;
select username,(uid+gid)/2 as pjcj from usertab;
11.算数运算函数
对集合中的各参数求和：sum(字段名)
集合的平均值：avg(字段名)
集合中的最小值：min(字段名)
集合中的最大值：max(字段名)
记录的个数：count(字段名)
样例
select sum(字段名) from 表；
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