前言：
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
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
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
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
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
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_salfrom emp where job='salesman'group by deptno ,jobhaving 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：
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
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
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
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
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
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