SQL 表查询之内连接

Posted 2021-03-05 萌新想吃鱼

 1 # 查询每名员工的部门信息，用ON子句规定表连接条件
 2 SELECT e.empno,e.ename,d.dname FROM t_emp e JOIN t_dept d ON e.deptno=d.deptno; 
 3 # 多种内连接写法
 4 SELECT e.empno,e.ename,d.dname FROM t_emp e JOIN t_dept d WHERE e.deptno=d.deptno; 
 5 SELECT e.empno,e.ename,d.dname FROM t_emp e , t_dept d WHERE e.deptno=d.deptno; 
 6 # 查询每个员工工号，姓名，部门名称，底薪，职位，工资等级
 7 SELECT e.empno,e.ename,d.dname,e.sal,e.job,s.grade FROM t_emp e,t_dept d,t_salgrade s WHERE e.deptno=d.deptno AND e.sal BETWEEN s.losal AND s.hisal ORDER BY s.grade;
 8 # 查找与SCOTT部门相同的员工都有谁
 9 # 子查询方式
10 SELECT ename FROM t_emp WHERE deptno=(SELECT deptno FROM t_emp WHERE ename="SCOTT") AND ename!="SCOTT";
11 # 表连接方式
12 SELECT e2.ename FROM t_emp e1 JOIN t_emp e2 ON e1.deptno=e2.deptno AND e2.ename!="SCOTT" WHERE e1.ename="SCOTT";
13 # 查询底薪超过公司平均底薪的员工信息
14 # 错误方式 SELECT e2.empno,e2.ename,e2.sal FROM t_emp e1 JOIN t_emp e2 ON e2.sal>AVG(e1.sal)
15 # 将结果集作为一张表来与其他表做表连接
16 SELECT e.empno,e.ename,e.sal FROM t_emp e JOIN (SELECT AVG(sal) avg FROM t_emp) t ON e.sal>=t.avg;
17 # 查询RESEARCH部门的人数、最高底薪、最低底薪、平均底薪、平均工龄(FLOOR强制舍位，CELL强制进位)
18 SELECT COUNT(*),AVG(e.sal),MAX(e.sal),MIN(e.sal),FLOOR(AVG(DATEDIFF(NOW(),e.hiredate)/365)) avghire 
　　FROM t_emp e JOIN t_dept d ON d.deptno=e.deptno WHERE d.dname="RESEARCH";
19 # 查询每种职业的最高工资，最低工资，平均工资，最高工资等级，最低工资等级
20 SELECT e.job,COUNT(*),MAX(e.sal+IFNULL(e.comm,0)),MIN(e.sal+IFNULL(e.comm,0)),AVG(e.sal+IFNULL(e.comm,0)),MAX(s.grade),MIN(s.grade) 
　　FROM t_emp e JOIN t_salgrade s ON e.sal+IFNULL(e.comm,0) BETWEEN s.losal AND s.hisal GROUP BY e.job;
21 # 查询每个超过部门平均底薪的员工信息(将部门平均底薪和部门编号结果集作为一个表去连接)
22 SELECT e.deptno,e.ename,e.sal,t.avg FROM t_emp e JOIN (SELECT AVG(sal) avg,deptno FROM t_emp GROUP BY deptno) t 
　　ON e.deptno=t.deptno AND e.sal>=t.avg;