MySQL 复合查询核心指南之多表、子查询与实战技巧

-- 1. 条件筛选：工资>500或岗位为manager，且姓名首字母为J
select * from emp where (sal>500 or job='manager') and ename like 'j%';
-- 2. 多字段排序：部门号升序，工资降序
select * from emp order by deptno asc, sal desc;
-- 3. 计算字段+排序：年薪（sal*12+补贴）降序
select ename, sal*12+ifnull(comm,0) as 年薪 from emp order by 年薪 desc;
-- 4. 聚合查询+筛选：各部门平均工资（保留2位小数）和最高工资
select deptno, format(avg(sal),2) as 平均工资, max(sal) as 最高工资 from emp group by deptno;
-- 5. having筛选聚合结果：平均工资低于2000的部门
select deptno, avg(sal) as avg_sal from emp group by deptno having avg_sal < 2000;

select 表1.字段, 表2.字段 
from 表1, 表2 
where 表1.关联字段 = 表2.关联字段 [and 其他筛选条件];

-- 核心：通过deptno关联emp和dept表，消除笛卡尔积
select emp.ename, emp.sal, dept.dname 
from emp, dept 
where emp.deptno = dept.deptno;

select emp.ename, emp.sal, dept.dname 
from emp, dept 
where emp.deptno = dept.deptno 
  and dept.deptno = 10; -- 筛选10号部门

select emp.ename, emp.sal, salgrade.grade 
from emp, salgrade 
where emp.sal between salgrade.losal and salgrade.hisal; -- 工资在薪资等级区间内

select 表别名1.字段, 表别名2.字段 
from 表 表别名1, 表 表别名2 
where 表别名1.关联字段 = 表别名2.关联字段 [and 筛选条件];

-- 方法1：子查询（简单场景）
select empno, ename from emp 
where empno = (select mgr from emp where ename='ford');
-- 方法2：自连接（复杂场景更灵活）
select leader.empno as 领导编号, leader.ename as 领导姓名 
from emp leader, emp worker -- leader=领导表，worker=员工表
where leader.empno = worker.mgr -- 领导编号=员工的上级编号
  and worker.ename = 'ford'; -- 筛选员工为ford

select * from emp 
where deptno = (select deptno from emp where ename='smith') -- 子查询返回smith的部门号
  and ename != 'smith'; -- 排除smith本人

select ename, job, sal, deptno from emp 
where job in (select distinct job from emp where deptno=10) -- 子查询返回10号部门的所有岗位
  and deptno != 10; -- 排除10号部门

select ename, sal, deptno from emp 
where sal > all(select sal from emp where deptno=30); -- 工资>30号部门所有员工工资

select ename, sal, deptno from emp 
where sal > any(select sal from emp where deptno=30); -- 工资>30号部门至少一个员工工资

select ename from emp 
where (deptno, job) = (select deptno, job from emp where ename='smith') -- 匹配部门+岗位组合
  and ename != 'smith';

select emp.ename, emp.deptno, emp.sal, format(tmp.asal,2) as 部门平均工资 
from emp, 
     (select avg(sal) as asal, deptno as dt from emp group by deptno) tmp -- 临时表：各部门平均工资
where emp.deptno = tmp.dt -- 员工部门=临时表部门
  and emp.sal > tmp.asal; -- 员工工资>部门平均工资

select emp.ename, emp.sal, emp.deptno, tmp.ms as 部门最高工资 
from emp, 
     (select max(sal) as ms, deptno from emp group by deptno) tmp -- 临时表：各部门最高工资
where emp.deptno = tmp.deptno 
  and emp.sal = tmp.ms;

-- 去重合并（自动删除重复行）
select 字段 from 表1 where 条件
union
select 字段 from 表2 where 条件;
-- 不去重合并（保留重复行，性能更优）
select 字段 from 表1 where 条件
union all
select 字段 from 表2 where 条件;

select ename, sal, job from emp where sal>2500
union -- 自动去重（manager中工资>2500的员工只显示一次）
select ename, sal, job from emp where job='manager';

select ename, sal, job from emp where sal>2500
union all -- 保留重复行（manager中工资>2500的员可能会显示多次）
select ename, sal, job from emp where job='manager';

select e.emp_no, s.salary 
from employees e, salaries s 
where e.emp_no = s.emp_no 
  and s.from_date = e.hire_date 
order by e.emp_no desc;

select concat('select count(*) from ', table_name, ';') as count_sql 
from information_schema.tables 
where table_schema = 'your_database_name'; -- 替换为你的数据库名

select emp_no from employees
where emp_no not in(select emp_no from dept_manager);

select e.emp_no, d.emp_no as manager 
from dept_emp as e, dept_manager as d 
where e.dept_no = d.dept_no 
  and e.emp_no != d.emp_no;