本篇文章主要简介下MySQL中where,group by ,order by ,limit,join,union ,union all,子表等查询语法。

测试数据准备
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create table emp (
    empno numeric(4) not null,
    ename varchar(10),
    job varchar(9),
    mgr numeric(4),
    hiredate datetime,
    sal numeric(7, 2),
    comm numeric(7, 2),
    deptno numeric(2)
);

create table dept (
    deptno numeric(2),
    dname varchar(14),
    loc varchar(13)
);

create table salgrade (
    grade numeric,
    losal numeric,
    hisal numeric
);

insert into dept values (10, 'ACCOUNTING', 'NEW YORK');
insert into dept values (20, 'RESEARCH', 'DALLAS');
insert into dept values (30, 'SALES', 'CHICAGO');
insert into dept values (40, 'OPERATIONS', 'BOSTON');

insert into salgrade values (1, 700, 1200);
insert into salgrade values (2, 1201, 1400);
insert into salgrade values (3, 1401, 2000);
insert into salgrade values (4, 2001, 3000);
insert into salgrade values (5, 3001, 9999);

insert into emp values (7369, 'SMITH', 'CLERK', 7902, '1980-12-17', 800, null, 20);
insert into emp values (7499, 'ALLEN', 'SALESMAN', 7698, '1981-02-20', 1600, 300, 30);
insert into emp values (7521, 'WARD', 'SALESMAN', 7698, '1981-02-22', 1250, 500, 30);
insert into emp values (7566, 'JONES', 'MANAGER', 7839, '1981-04-02', 2975, null, 20);
insert into emp values (7654, 'MARTIN', 'SALESMAN', 7698, '1981-09-28', 1250, 1400, 30);
insert into emp values (7698, 'BLAKE', 'MANAGER', 7839, '1981-05-01', 2850, null, 30);
insert into emp values (7782, 'CLARK', 'MANAGER', 7839, '1981-06-09', 2450, null, 10);
insert into emp values (7788, 'SCOTT', 'ANALYST', 7566, '1982-12-09', 3000, null, 20);
insert into emp values (7839, 'KING', 'PRESIDENT', null, '1981-11-17', 5000, null, 10);
insert into emp values (7844, 'TURNER', 'SALESMAN', 7698, '1981-09-08', 1500, 0, 30);
insert into emp values (7876, 'ADAMS', 'CLERK', 7788, '1983-01-12', 1100, null, 20);
insert into emp values (7900, 'JAMES', 'CLERK', 7698, '1981-12-03', 950, null, 30);
insert into emp values (7902, 'FORD', 'ANALYST', 7566, '1981-12-03', 3000, null, 20);
insert into emp values (7934, 'MILLER', 'CLERK', 7782, '1982-01-23', 1300, null, 10);
1.模糊查询
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mysql> select * from emp where ename like '%S%'; 
+-------+-------+---------+------+---------------------+---------+------+--------+
| empno | ename | job     | mgr  | hiredate            | sal     | comm | deptno |
+-------+-------+---------+------+---------------------+---------+------+--------+
|  7369 | SMITH | CLERK   | 7902 | 1980-12-17 00:00:00 |  800.00 | NULL |     20 |
|  7566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
|  7788 | SCOTT | ANALYST | 7566 | 1982-12-09 00:00:00 | 3000.00 | NULL |     20 |
|  7876 | ADAMS | CLERK   | 7788 | 1983-01-12 00:00:00 | 1100.00 | NULL |     20 |
|  7900 | JAMES | CLERK   | 7698 | 1981-12-03 00:00:00 |  950.00 | NULL |     30 |
+-------+-------+---------+------+---------------------+---------+------+--------+
5 rows in set (0.00 sec)

mysql> select * from emp where ename like 'S%'; 
+-------+-------+---------+------+---------------------+---------+------+--------+
| empno | ename | job     | mgr  | hiredate            | sal     | comm | deptno |
+-------+-------+---------+------+---------------------+---------+------+--------+
|  7369 | SMITH | CLERK   | 7902 | 1980-12-17 00:00:00 |  800.00 | NULL |     20 |
|  7788 | SCOTT | ANALYST | 7566 | 1982-12-09 00:00:00 | 3000.00 | NULL |     20 |
+-------+-------+---------+------+---------------------+---------+------+--------+
2 rows in set (0.01 sec)

mysql> select * from emp where ename like '%S';
+-------+-------+---------+------+---------------------+---------+------+--------+
| empno | ename | job     | mgr  | hiredate            | sal     | comm | deptno |
+-------+-------+---------+------+---------------------+---------+------+--------+
|  7566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
|  7876 | ADAMS | CLERK   | 7788 | 1983-01-12 00:00:00 | 1100.00 | NULL |     20 |
|  7900 | JAMES | CLERK   | 7698 | 1981-12-03 00:00:00 |  950.00 | NULL |     30 |
+-------+-------+---------+------+---------------------+---------+------+--------+
3 rows in set (0.00 sec)

mysql> select * from emp where ename like '_O%';
+-------+-------+---------+------+---------------------+---------+------+--------+
| empno | ename | job     | mgr  | hiredate            | sal     | comm | deptno |
+-------+-------+---------+------+---------------------+---------+------+--------+
|  7566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
|  7902 | FORD  | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL |     20 |
+-------+-------+---------+------+---------------------+---------+------+--------+
2 rows in set (0.00 sec)
总结：
%表示任意0个或多个字符，可匹配任意类型和长度的字符；
_表示任意单个字符，匹配单个任意字符。
2.排序
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mysql> select * from emp order by  sal;
+-------+--------+-----------+------+---------------------+---------+---------+--------+
| empno | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno |
+-------+--------+-----------+------+---------------------+---------+---------+--------+
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |
|  7876 | ADAMS  | CLERK     | 7788 | 1983-01-12 00:00:00 | 1100.00 |    NULL |     20 |
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |
|  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |
|  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |
|  7788 | SCOTT  | ANALYST   | 7566 | 1982-12-09 00:00:00 | 3000.00 |    NULL |     20 |
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |
|  7839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |
+-------+--------+-----------+------+---------------------+---------+---------+--------+
14 rows in set (0.00 sec)

mysql> select * from emp order by  sal asc;
+-------+--------+-----------+------+---------------------+---------+---------+--------+
| empno | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno |
+-------+--------+-----------+------+---------------------+---------+---------+--------+
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |
|  7876 | ADAMS  | CLERK     | 7788 | 1983-01-12 00:00:00 | 1100.00 |    NULL |     20 |
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |
|  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |
|  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |
|  7788 | SCOTT  | ANALYST   | 7566 | 1982-12-09 00:00:00 | 3000.00 |    NULL |     20 |
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |
|  7839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |
+-------+--------+-----------+------+---------------------+---------+---------+--------+
14 rows in set (0.00 sec)

mysql> select * from emp order by  sal desc;
+-------+--------+-----------+------+---------------------+---------+---------+--------+
| empno | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno |
+-------+--------+-----------+------+---------------------+---------+---------+--------+
|  7839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |
|  7788 | SCOTT  | ANALYST   | 7566 | 1982-12-09 00:00:00 | 3000.00 |    NULL |     20 |
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |
|  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |
|  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |
|  7876 | ADAMS  | CLERK     | 7788 | 1983-01-12 00:00:00 | 1100.00 |    NULL |     20 |
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |
+-------+--------+-----------+------+---------------------+---------+---------+--------+
14 rows in set (0.00 sec)
总结：
order by排序默认按asc升序来排列
也可指定desc降序排列
3.限制多少行
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mysql> select * from emp limit  3;
+-------+-------+----------+------+---------------------+---------+--------+--------+
| empno | ename | job      | mgr  | hiredate            | sal     | comm   | deptno |
+-------+-------+----------+------+---------------------+---------+--------+--------+
|  7369 | SMITH | CLERK    | 7902 | 1980-12-17 00:00:00 |  800.00 |   NULL |     20 |
|  7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 |     30 |
|  7521 | WARD  | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 |     30 |
+-------+-------+----------+------+---------------------+---------+--------+--------+
3 rows in set (0.00 sec)

mysql> select * from emp order by  sal desc limit  3;
+-------+-------+-----------+------+---------------------+---------+------+--------+
| empno | ename | job       | mgr  | hiredate            | sal     | comm | deptno |
+-------+-------+-----------+------+---------------------+---------+------+--------+
|  7839 | KING  | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL |     10 |
|  7902 | FORD  | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL |     20 |
|  7788 | SCOTT | ANALYST   | 7566 | 1982-12-09 00:00:00 | 3000.00 | NULL |     20 |
+-------+-------+-----------+------+---------------------+---------+------+--------+
3 rows in set (0.00 sec)
总结：
limit限定显示前多少行，可与order by联合使用
4.聚合函数
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count() sum()函数用法：
#1.各个部门的薪水和
mysql> select deptno,sum(sal) from  emp group by deptno;
+--------+----------+
| deptno | sum(sal) |
+--------+----------+
|     10 |  8750.00 |
|     20 | 10875.00 |
|     30 |  9400.00 |
+--------+----------+
3 rows in set (0.01 sec)

#2.group by字段必须出现在select字段后面   各个部门的各个岗位的薪水和
mysql> select deptno,job, sum(sal) from  emp group by deptno ,job;
+--------+-----------+----------+
| deptno | job       | sum(sal) |
+--------+-----------+----------+
|     10 | CLERK     |  1300.00 |
|     10 | MANAGER   |  2450.00 |
|     10 | PRESIDENT |  5000.00 |
|     20 | ANALYST   |  6000.00 |
|     20 | CLERK     |  1900.00 |
|     20 | MANAGER   |  2975.00 |
|     30 | CLERK     |   950.00 |
|     30 | MANAGER   |  2850.00 |
|     30 | SALESMAN  |  5600.00 |
+--------+-----------+----------+
9 rows in set (0.01 sec)

#3.having    薪水和>5000的各个部门的各个岗位
mysql> select deptno,job,  sum(sal) 
    -> from  emp 
    -> group by deptno ,job
    -> having  sum(sal)>5000; 
+--------+----------+----------+
| deptno | job      | sum(sal) |
+--------+----------+----------+
|     20 | ANALYST  |  6000.00 |
|     30 | SALESMAN |  5600.00 |
+--------+----------+----------+
2 rows in set (0.00 sec)
#4.常用组合where  order  limit 
select deptno,job,  sum(sal) as sum_sal
from  emp 
where job='SALESMAN'
group by deptno ,job
having  sum(sal)>5000 
order by sum(sal) desc 
limit 1;

下面介绍下join及union的用法 数据准备：

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create table testa(aid int,aname varchar(40));

create table testb(bid int,bname varchar(40),age int);

insert into testa values(1,'xiaoming');
insert into testa values(2,'LY');
insert into testa values(3,'KUN');
insert into testa values(4,'ZIDONG');
insert into testa values(5,'HB');


insert into testb values(1,'xiaoming',10);
insert into testb values(2,'LY',100);
insert into testb values(3,'KUN',200);
insert into testb values(4,'ZIDONG',110);
insert into testb values(6,'niu',120);
insert into testb values(7,'meng',130);
insert into testb values(8,'mi',170);
5.left join
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mysql> select
    -> a.aid,a.aname,
    -> b.bid,b.bname,b.age
    -> from testa as a
    -> left join testb as b on a.aid=b.bid; 
+------+----------+------+----------+------+
| aid  | aname    | bid  | bname    | age  |
+------+----------+------+----------+------+
|    1 | xiaoming |    1 | xiaoming |   10 |
|    2 | LY       |    2 | LY       |  100 |
|    3 | KUN      |    3 | KUN      |  200 |
|    4 | ZIDONG   |    4 | ZIDONG   |  110 |
|    5 | HB       | NULL | NULL     | NULL |
+------+----------+------+----------+------+
5 rows in set (0.00 sec)
总结：
a left join b  a表全，用b表去匹配a表
LEFT JOIN 关键字会从左表 (a) 那里返回所有的行，即使在右表 (b) 中没有匹配的行,匹配不到的列用NULL代替
6.right join
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mysql> select
    -> a.aid,a.aname,
    -> b.bid,b.bname,b.age
    -> from testa as a
    -> right join testb as b on a.aid=b.bid;
+------+----------+------+----------+------+
| aid  | aname    | bid  | bname    | age  |
+------+----------+------+----------+------+
|    1 | xiaoming |    1 | xiaoming |   10 |
|    2 | LY       |    2 | LY       |  100 |
|    3 | KUN      |    3 | KUN      |  200 |
|    4 | ZIDONG   |    4 | ZIDONG   |  110 |
| NULL | NULL     |    6 | niu      |  120 |
| NULL | NULL     |    7 | meng     |  130 |
| NULL | NULL     |    8 | mi       |  170 |
+------+----------+------+----------+------+
7 rows in set (0.00 sec)
总结：
a right join b  b表全，用a表去匹配b表
RIGHT JOIN 关键字会右表 (b) 那里返回所有的行，即使在左表 (a) 中没有匹配的行,匹配不到的列用NULL代替
7.inner join
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mysql> select
    -> a.aid,a.aname,
    -> b.bid,b.bname,b.age
    -> from testa as a
    -> inner join testb as b on a.aid=b.bid; 
+------+----------+------+----------+------+
| aid  | aname    | bid  | bname    | age  |
+------+----------+------+----------+------+
|    1 | xiaoming |    1 | xiaoming |   10 |
|    2 | LY       |    2 | LY       |  100 |
|    3 | KUN      |    3 | KUN      |  200 |
|    4 | ZIDONG   |    4 | ZIDONG   |  110 |
+------+----------+------+----------+------+
4 rows in set (0.00 sec)

mysql> select
    -> a.aid,a.aname,
    -> b.bid,b.bname,b.age
    -> from testa as a
    -> join testb as b on a.aid=b.bid; 
+------+----------+------+----------+------+
| aid  | aname    | bid  | bname    | age  |
+------+----------+------+----------+------+
|    1 | xiaoming |    1 | xiaoming |   10 |
|    2 | LY       |    2 | LY       |  100 |
|    3 | KUN      |    3 | KUN      |  200 |
|    4 | ZIDONG   |    4 | ZIDONG   |  110 |
+------+----------+------+----------+------+
4 rows in set (0.00 sec)
总结：
inner join 与join 效果一样
在表中存在至少一个匹配时，INNER JOIN 关键字返回行
8.union与union all
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mysql> select aid,aname from testa
    -> union
    -> select bid,bname from testb;
+------+----------+
| aid  | aname    |
+------+----------+
|    1 | xiaoming |
|    2 | LY       |
|    3 | KUN      |
|    4 | ZIDONG   |
|    5 | HB       |
|    6 | niu      |
|    7 | meng     |
|    8 | mi       |
+------+----------+
8 rows in set (0.01 sec)

mysql> select aid,aname from testa
    -> union all
    -> select bid,bname from testb;
+------+----------+
| aid  | aname    |
+------+----------+
|    1 | xiaoming |
|    2 | LY       |
|    3 | KUN      |
|    4 | ZIDONG   |
|    5 | HB       |
|    1 | xiaoming |
|    2 | LY       |
|    3 | KUN      |
|    4 | ZIDONG   |
|    6 | niu      |
|    7 | meng     |
|    8 | mi       |
+------+----------+
12 rows in set (0.00 sec)
总结：
union 会去重 union all不去重