MySql数据库单表、多表查询练习

– 按部门求出该部门平均工资分组查询
SELECT TRUNCATE(AVG(SAL),0) AS ‘部门平均工资’ FROM emp GROUP BY emp.DEPTNO;

– 查询部门平均工资大于2000元的部门
SELECT dept.DNAME AS ‘平均工资大于2000的部门’ FROM dept WHERE dept.DEPTNO IN (SELECT emp.DEPTNO FROM emp GROUP BY emp.DEPTNO HAVING TRUNCATE(AVG(SAL),0) > 2000);

– 按部门平均工资降序排列
SELECT TRUNCATE(AVG(SAL),0) AS ‘部门平均工资’ , emp.DEPTNO FROM emp GROUP BY emp.DEPTNO ORDER BY TRUNCATE(AVG(SAL),0) DESC;

– 除10号部门外，查询部门平均工资大于2000元的部门
SELECT AVG(SAL) FROM emp WHERE emp.DEPTNO != 10 GROUP BY emp.DEPTNO HAVING AVG(SAL) > 2000;

– 显示部门平均工资的最大值
SELECT TRUNCATE(AVG(SAL),0) AS ‘部门平均工资最大值’, emp.DEPTNO FROM emp GROUP BY emp.DEPTNO ORDER BY TRUNCATE(AVG(SAL),0) DESC LIMIT 0,1;

– 查询员工的编号，姓名，部门名，使用表别名简化
SELECT emp.EMPNO, emp.ENAME, dept.DNAME FROM emp, dept WHERE emp.DEPTNO = dept.DEPTNO;

– 显示员工的编号，姓名，月薪，工资级别
SELECT emp.EMPNO, emp.ENAME, emp.SAL, salgrade.GRADE FROM emp, salgrade WHERE emp.SAL BETWEEN salgrade.LOSAL AND salgrade.HISAL ORDER BY salgrade.GRADE;

– 按部门10,20,30,40号，统计各部门员工人数，要求显示部门号，部门名，人数
SELECT dept.DEPTNO, dept.DNAME, COUNT(emp.EMPNO) FROM dept, emp WHERE dept.DEPTNO = emp.DEPTNO GROUP BY emp.DEPTNO; # 因为一个部门没有数据，所以没有显示全
SELECT dept.DEPTNO, dept.DNAME, COUNT(emp.EMPNO) FROM dept LEFT JOIN emp ON dept.DEPTNO = emp.DEPTNO GROUP BY emp.DEPTNO; # 至于分清左右，主要看全显示的是哪个表，是哪个就朝那边

– 显示每个员工的上级
SELECT a.ENAME, b.ENAME FROM emp a LEFT JOIN emp b ON b.EMPNO = a.MGR;
SELECT CONCAT(a.ENAME,‘的BOSS是–>’,b.ENAME) FROM emp a LEFT JOIN emp b ON b.EMPNO = a.MGR;

– 查询员工薪资最高的员工信息
SELECT * FROM emp WHERE emp.SAL = (SELECT MAX(SAL) FROM emp);

– 查询工资比WARD高的员工信息
SELECT * FROM emp WHERE emp.SAL > (SELECT SAL FROM emp WHERE emp.ENAME =‘WARD’);

– 查询部门名为’SALES’的员工信息
– 方式一：子查询
SELECT * FROM emp WHERE emp.DEPTNO = (SELECT dept.DEPTNO FROM dept WHERE dept.DNAME =‘SALES’);
– 方式二：多表查询
SELECT emp.* FROM emp, dept WHERE emp.DEPTNO = dept.DEPTNO AND dept.DNAME = ‘SALES’;

– 查询部门名为’ACCOUNTING’或’SALES’的员工信息（多行子查询，使用in关键字）
SELECT * FROM emp WHERE emp.DEPTNO IN (SELECT dept.DEPTNO FROM dept WHERE dept.DNAME = ‘ACCOUNTING’ OR dept.DNAME = ‘SALES’);

– 查询部门名不是’ACCOUNTING’或’SALES’的员工信息（多行子查询，使用in关键字）
SELECT * FROM emp WHERE emp.DEPTNO NOT IN (SELECT dept.DEPTNO FROM dept WHERE dept.DNAME = ‘ACCOUNTING’ OR dept.DNAME = ‘SALES’);

– 查询工资比20号部门【任意any】一个员工工资【低<】的员工信息（多行子查询，使用any关键字）
SELECT * FROM emp WHERE emp.SAL < ANY(SELECT SAL FROM emp WHERE emp.DEPTNO = 20);
SELECT * FROM emp WHERE emp.SAL < (SELECT MAX(SAL) FROM emp WHERE emp.DEPTNO = 20);

– 查询工资比30号部门【所有all】员工【低<】的员工信息（多行子查询，使用all关键字）
SELECT * FROM emp WHERE emp.SAL < ALL(SELECT SAL FROM emp WHERE emp.DEPTNO = 30);
SELECT * FROM emp WHERE emp.SAL < (SELECT MIN(SAL) FROM emp WHERE emp.DEPTNO = 30);

– 使用并集运算，查询20号部门或30号部门的员工信息
SELECT * FROM emp WHERE emp.DEPTNO = 20
UNION
SELECT * FROM emp WHERE emp.DEPTNO = 30;
– union：二个集合中，如果都有相同的，取其一
– union all：二个集合中，如果都有相同的，都取

– 使用交集运算[intersect]，查询工资在1000-2000和1500-2500之间的员工信息（方式一）
– select * from emp where sal between 1000 and 2000
– intersect
– select * from emp where sal between 1500 and 2500;
– （方式二：）
SELECT * FROM emp WHERE (emp.SAL BETWEEN 1000 AND 2000) AND (emp.SAL BETWEEN 1500 AND 2500);

– 使用差集运算[minus]，查询工资在1000-2000，但不在1500-2500之间的员工信息（方式一）
– select * from emp where sal between 1000 and 2000
– minus
– select * from emp where sal between 1500 and 2500;
– （方式二：）
SELECT * FROM emp WHERE (emp.SAL BETWEEN 1000 AND 2000) AND (emp.SAL NOT BETWEEN 1500 AND 2500);

集合查询的细节：
1）集合操作时，必须确保集合列数是相等
select empno,ename,sal,comm from emp where deptno = 20
union
select empno,ename,sal from emp where deptno = 30;错

2）集合操作时，必须确保集合列类型对应相同
select empno,ename,sal,comm from emp where deptno = 20
union
select empno,ename,sal,hiredate from emp where deptno = 30;错

3）A union B union C = C union B union A（交换律）
select * from emp where deptno = 10
union
select * from emp where deptno = 20
union
select * from emp where deptno = 30;

4）当多个集合操作时，结果的列名由第一个集合列名决定
select empno “编号”,ename “姓名”,sal “薪水” from emp where deptno = 20
union
select empno,ename,sal from emp where deptno = 10;