显示每个高于自己部门平均工资的员工的姓名、部门、工资、平均工资
sql
mysql> select avg(sal) from emp;
+-------------+
| avg(sal) |
+-------------+
| 2073.214286 |
+-------------+
1 row in set (0.00 sec)
mysql> select avg(sal) from emp group by deptno;
+-------------+
| avg(sal) |
+-------------+
| 2916.666667 |
| 2175.000000 |
| 1566.666667 |
+-------------+
3 rows in set (0.00 sec)
mysql> select deptno,avg(sal) from emp group by deptno;
+--------+-------------+
| deptno | avg(sal) |
+--------+-------------+
| 10 | 2916.666667 |
| 20 | 2175.000000 |
| 30 | 1566.666667 |
+--------+-------------+
3 rows in set (0.00 sec)
mysql> select * from emp,(select deptno,avg(sal) from emp group by deptno);
ERROR 1248 (42000): Every derived table must have its own alias
mysql> select * from emp,(select deptno,avg(sal) from emp group by deptno) tmp;
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+-------------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno | deptno | avg(sal) |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+-------------+
| 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 10 | 2916.666667 |
| 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 10 | 2916.666667 |
| 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 10 | 2916.666667 |
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 10 | 2916.666667 |
| 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 10 | 2916.666667 |
| 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 10 | 2916.666667 |
| 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 10 | 2916.666667 |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 10 | 2916.666667 |
| 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 10 | 2916.666667 |
| 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 10 | 2916.666667 |
| 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 10 | 2916.666667 |
| 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 10 | 2916.666667 |
| 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 10 | 2916.666667 |
| 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 10 | 2916.666667 |
| 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 20 | 2175.000000 |
| 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 20 | 2175.000000 |
| 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 20 | 2175.000000 |
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 20 | 2175.000000 |
| 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 20 | 2175.000000 |
| 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 20 | 2175.000000 |
| 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 20 | 2175.000000 |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 20 | 2175.000000 |
| 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 20 | 2175.000000 |
| 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 20 | 2175.000000 |
| 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 20 | 2175.000000 |
| 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 20 | 2175.000000 |
| 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 20 | 2175.000000 |
| 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 20 | 2175.000000 |
| 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 30 | 1566.666667 |
| 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 30 | 1566.666667 |
| 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 30 | 1566.666667 |
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 30 | 1566.666667 |
| 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 30 | 1566.666667 |
| 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 30 | 1566.666667 |
| 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 30 | 1566.666667 |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 30 | 1566.666667 |
| 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 30 | 1566.666667 |
| 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 30 | 1566.666667 |
| 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 30 | 1566.666667 |
| 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 30 | 1566.666667 |
| 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 30 | 1566.666667 |
| 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 30 | 1566.666667 |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+-------------+
42 rows in set (0.00 sec)
mysql> select * from emp,(select deptno,avg(sal) from emp group by deptno) tmp where emp.deptno=tmp.deptno;
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+-------------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno | deptno | avg(sal) |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+-------------+
| 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 20 | 2175.000000 |
| 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 30 | 1566.666667 |
| 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 30 | 1566.666667 |
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 20 | 2175.000000 |
| 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 30 | 1566.666667 |
| 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 30 | 1566.666667 |
| 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 10 | 2916.666667 |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 20 | 2175.000000 |
| 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 10 | 2916.666667 |
| 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 30 | 1566.666667 |
| 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 20 | 2175.000000 |
| 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 30 | 1566.666667 |
| 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 20 | 2175.000000 |
| 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 10 | 2916.666667 |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+-------------+
14 rows in set (0.00 sec)
mysql> select * from emp,(select deptno,avg(sal) as myavg from emp group by deptno) tmp where emp.deptno=tmp.deptno;
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+-------------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno | deptno | myavg |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+-------------+
| 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 20 | 2175.000000 |
| 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 30 | 1566.666667 |
| 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 30 | 1566.666667 |
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 20 | 2175.000000 |
| 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 30 | 1566.666667 |
| 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 30 | 1566.666667 |
| 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 10 | 2916.666667 |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 20 | 2175.000000 |
| 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 10 | 2916.666667 |
| 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 30 | 1566.666667 |
| 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 20 | 2175.000000 |
| 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 30 | 1566.666667 |
| 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 20 | 2175.000000 |
| 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 10 | 2916.666667 |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+-------------+
14 rows in set (0.00 sec)
mysql> select * from emp,(select deptno,avg(sal) as myavg from emp group by deptno) tmp where emp.deptno=tmp.deptno and emp.sal>tmp.myavg;
+--------+-------+-----------+------+---------------------+---------+--------+--------+--------+-------------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno | deptno | myavg |
+--------+-------+-----------+------+---------------------+---------+--------+--------+--------+-------------+
| 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 30 | 1566.666667 |
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 20 | 2175.000000 |
| 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 30 | 1566.666667 |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 20 | 2175.000000 |
| 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 10 | 2916.666667 |
| 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 20 | 2175.000000 |
+--------+-------+-----------+------+---------------------+---------+--------+--------+--------+-------------+
6 rows in set (0.00 sec)
mysql> select ename,emp.deptno from emp,(select deptno,avg(sal) as myavg from emp group by deptno) tmp where emp.deptno=tmp.deptno and emp.sal>tmp.myavg;
+-------+--------+
| ename | deptno |
+-------+--------+
| ALLEN | 30 |
| JONES | 20 |
| BLAKE | 30 |
| SCOTT | 20 |
| KING | 10 |
| FORD | 20 |
+-------+--------+
6 rows in set (0.00 sec)
mysql> select *from dept;
+--------+------------+----------+
| deptno | dname | loc |
+--------+------------+----------+
| 10 | ACCOUNTING | NEW YORK |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+------------+----------+
4 rows in set (0.00 sec)
mysql> select *from dept ,(select ename,emp.deptno from emp,(select deptno,avg(sal) as myavg from emp group by deptno) tmp where emp.ddeptno=tmp.deptno and emp.sal>tmp.myavg) t1;
+--------+------------+----------+-------+--------+
| deptno | dname | loc | ename | deptno |
+--------+------------+----------+-------+--------+
| 10 | ACCOUNTING | NEW YORK | ALLEN | 30 |
| 20 | RESEARCH | DALLAS | ALLEN | 30 |
| 30 | SALES | CHICAGO | ALLEN | 30 |
| 40 | OPERATIONS | BOSTON | ALLEN | 30 |
| 10 | ACCOUNTING | NEW YORK | JONES | 20 |
| 20 | RESEARCH | DALLAS | JONES | 20 |
| 30 | SALES | CHICAGO | JONES | 20 |
| 40 | OPERATIONS | BOSTON | JONES | 20 |
| 10 | ACCOUNTING | NEW YORK | BLAKE | 30 |
| 20 | RESEARCH | DALLAS | BLAKE | 30 |
| 30 | SALES | CHICAGO | BLAKE | 30 |
| 40 | OPERATIONS | BOSTON | BLAKE | 30 |
| 10 | ACCOUNTING | NEW YORK | SCOTT | 20 |
| 20 | RESEARCH | DALLAS | SCOTT | 20 |
| 30 | SALES | CHICAGO | SCOTT | 20 |
| 40 | OPERATIONS | BOSTON | SCOTT | 20 |
| 10 | ACCOUNTING | NEW YORK | KING | 10 |
| 20 | RESEARCH | DALLAS | KING | 10 |
| 30 | SALES | CHICAGO | KING | 10 |
| 40 | OPERATIONS | BOSTON | KING | 10 |
| 10 | ACCOUNTING | NEW YORK | FORD | 20 |
| 20 | RESEARCH | DALLAS | FORD | 20 |
| 30 | SALES | CHICAGO | FORD | 20 |
| 40 | OPERATIONS | BOSTON | FORD | 20 |
+--------+------------+----------+-------+--------+
24 rows in set (0.00 sec)
mysql> select *from dept ,(select ename,emp.deptno from emp,(select deptno,avg(sal) as myavg from emp group by deptno) tmp where emp.deptno=tmp.deptno and emp.sal>tmp.myavg) t1 where t1.deptno=dept.deptno;
+--------+------------+----------+-------+--------+
| deptno | dname | loc | ename | deptno |
+--------+------------+----------+-------+--------+
| 30 | SALES | CHICAGO | ALLEN | 30 |
| 20 | RESEARCH | DALLAS | JONES | 20 |
| 30 | SALES | CHICAGO | BLAKE | 30 |
| 20 | RESEARCH | DALLAS | SCOTT | 20 |
| 10 | ACCOUNTING | NEW YORK | KING | 10 |
| 20 | RESEARCH | DALLAS | FORD | 20 |
+--------+------------+----------+-------+--------+
6 rows in set (0.00 sec)
mysql> select t1.ename ,dept.loc,t1.deptno from dept ,(select ename,emp.deptno from emp,(select deptno,avg(sal) as myavg from emp grooup by deptno) tmp where emp.deptno=tmp.deptno and emp.sal>tmp.myavg) t1 where t1.deptno=dept.deptno;
+-------+----------+--------+
| ename | loc | deptno |
+-------+----------+--------+
| ALLEN | CHICAGO | 30 |
| JONES | DALLAS | 20 |
| BLAKE | CHICAGO | 30 |
| SCOTT | DALLAS | 20 |
| KING | NEW YORK | 10 |
| FORD | DALLAS | 20 |
+-------+----------+--------+
6 rows in set (0.00 sec)
mysql> 这一系列查询展示了如何使用子查询和分组聚合函数来筛选数据，并将其与其他表进行联结查询。
计算所有员工的平均薪水:
SELECT AVG(sal) FROM emp;
这条查询返回了所有员工的平均薪水。
按部门分组计算平均薪水:
SELECT deptno, AVG(sal) FROM emp GROUP BY deptno;
这条查询按部门分组并计算每个部门的平均薪水。
尝试直接联结员工表和分组查询结果但忘记为分组查询结果指定别名，导致错误:
SELECT * FROM emp, (SELECT deptno, AVG(sal) FROM emp GROUP BY deptno);
MySQL 报错，提示每个派生表必须有自己的别名。
为分组查询结果指定别名后进行联结查询:
SELECT * FROM emp, (SELECT deptno, AVG(sal) FROM emp GROUP BY deptno) AS tmp;
这条查询返回了员工表的所有记录与每个部门平均薪水的笛卡尔积。
基于部门号进行连接，筛选出员工的薪水高于其所在部门平均薪水的记录:
SELECT * FROM emp, (SELECT deptno, AVG(sal) AS myavg FROM emp GROUP BY deptno) AS tmp WHERE emp.deptno = tmp.deptno AND emp.sal > tmp.myavg;
这条查询找出了薪水高于其所在部门平均薪水的员工记录。
列出上述查询结果的员工名称、部门位置和部门编号:
SELECT t1.ename, dept.loc, t1.deptno FROM dept, (SELECT ename, emp.deptno FROM emp, (SELECT deptno, AVG(sal) AS myavg FROM emp GROUP BY deptno) AS tmp WHERE emp.deptno = tmp.deptno AND emp.sal > tmp.myavg) AS t1 WHERE t1.deptno = dept.deptno;
这条查询进一步联结了dept表，用以显示符合条件的员工名称、他们所在部门的位置和部门编号。
查询每个部门工资最高的人的姓名、工资、部门、最高工资
sql
mysql> select deptno ,max(sal) from emp;
ERROR 1140 (42000): In aggregated query without GROUP BY, expression #1 of SELECT list contains nonaggregated column 'scott.emp.deptno'; this is incompatible with sql_mode=only_full_group_by
mysql> select max(sal) from emp;
+----------+
| max(sal) |
+----------+
| 5000.00 |
+----------+
1 row in set (0.00 sec)
mysql> select max(sal) from emp group by deptno;
+----------+
| max(sal) |
+----------+
| 5000.00 |
| 3000.00 |
| 2850.00 |
+----------+
3 rows in set (0.00 sec)
mysql> select deptno,max(sal) from emp group by deptno;
+--------+----------+
| deptno | max(sal) |
+--------+----------+
| 10 | 5000.00 |
| 20 | 3000.00 |
| 30 | 2850.00 |
+--------+----------+
3 rows in set (0.00 sec)
mysql> select* from emp t1 ,(select deptno ,max(sal) from emp group by deptno ) t2;
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+----------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno | deptno | max(sal) |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+----------+
| 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 10 | 5000.00 |
| 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 10 | 5000.00 |
| 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 10 | 5000.00 |
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 10 | 5000.00 |
| 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 10 | 5000.00 |
| 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 10 | 5000.00 |
| 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 10 | 5000.00 |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 10 | 5000.00 |
| 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 10 | 5000.00 |
| 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 10 | 5000.00 |
| 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 10 | 5000.00 |
| 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 10 | 5000.00 |
| 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 10 | 5000.00 |
| 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 10 | 5000.00 |
| 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 20 | 3000.00 |
| 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 20 | 3000.00 |
| 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 20 | 3000.00 |
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 20 | 3000.00 |
| 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 20 | 3000.00 |
| 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 20 | 3000.00 |
| 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 20 | 3000.00 |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 20 | 3000.00 |
| 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 20 | 3000.00 |
| 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 20 | 3000.00 |
| 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 20 | 3000.00 |
| 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 20 | 3000.00 |
| 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 20 | 3000.00 |
| 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 20 | 3000.00 |
| 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 30 | 2850.00 |
| 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 30 | 2850.00 |
| 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 30 | 2850.00 |
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 30 | 2850.00 |
| 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 30 | 2850.00 |
| 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 30 | 2850.00 |
| 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 30 | 2850.00 |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 30 | 2850.00 |
| 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 30 | 2850.00 |
| 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 30 | 2850.00 |
| 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 30 | 2850.00 |
| 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 30 | 2850.00 |
| 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 30 | 2850.00 |
| 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 30 | 2850.00 |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+----------+
42 rows in set (0.00 sec)
mysql> select* from emp t1 ,(select deptno ,max(sal) from emp group by deptno ) t2 where t1.deptno =t2.deptno;
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+----------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno | deptno | max(sal) |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+----------+
| 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 20 | 3000.00 |
| 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 30 | 2850.00 |
| 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 30 | 2850.00 |
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 20 | 3000.00 |
| 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 30 | 2850.00 |
| 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 30 | 2850.00 |
| 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 10 | 5000.00 |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 20 | 3000.00 |
| 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 10 | 5000.00 |
| 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 30 | 2850.00 |
| 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 20 | 3000.00 |
| 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 30 | 2850.00 |
| 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 20 | 3000.00 |
| 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 10 | 5000.00 |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+----------+
14 rows in set (0.00 sec)
mysql> select* from emp t1 ,(select deptno ,max(sal) mymax from emp group by deptno ) t2 where t1.deptno =t2.deptno and t1.sal=t2.mymax;
+--------+-------+-----------+------+---------------------+---------+------+--------+--------+---------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno | deptno | mymax |
+--------+-------+-----------+------+---------------------+---------+------+--------+--------+---------+
| 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 30 | 2850.00 |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 20 | 3000.00 |
| 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 10 | 5000.00 |
| 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 20 | 3000.00 |
+--------+-------+-----------+------+---------------------+---------+------+--------+--------+---------+
4 rows in set (0.00 sec)
mysql> select ename,sal,t1.deptno,mymax from emp t1 ,(select deptno ,max(sal) mymax from emp group by deptno ) t2 where t1.deptno =t2.ddeptno and t1.sal=t2.mymax;
+-------+---------+--------+---------+
| ename | sal | deptno | mymax |
+-------+---------+--------+---------+
| BLAKE | 2850.00 | 30 | 2850.00 |
| SCOTT | 3000.00 | 20 | 3000.00 |
| KING | 5000.00 | 10 | 5000.00 |
| FORD | 3000.00 | 20 | 3000.00 |
+-------+---------+--------+---------+
4 rows in set (0.00 sec)
mysql> 这一系列的 MySQL 查询演示了如何使用聚合函数（如 MAX()）和分组（GROUP BY）来查询数据，并结合子查询和连接操作来进一步分析和筛选数据。
首先尝试查询每个部门的最高薪水，但没有使用 GROUP BY 语句，导致 MySQL 报错。这是因为在启用了 only_full_group_by SQL 模式的情况下，如果查询包含了聚合函数，那么所有非聚合列都必须包含在 GROUP BY 子句中。
查询了所有员工中的最高薪水，这是一个简单的聚合查询，返回了单一的最大值。
通过对每个部门进行分组，查询了每个部门的最高薪水。这展示了如何使用 GROUP BY 语句来分析每个分组（在这里是每个部门）的数据。
尝试联结员工表 (emp) 和一个子查询结果（每个部门的最高薪水），但由于没有指定连接条件，结果是一个笛卡尔积，这通常不是预期的结果。
修正了上述查询，通过添加 WHERE 子句来指定连接条件（t1.deptno = t2.deptno），确保只有当员工所在部门与子查询中的部门号相匹配时才返回记录。这个查询显示了每个部门及其最高薪水的员工信息。
最后，通过将薪水与每个部门的最高薪水相匹配（t1.sal = t2.mymax），筛选出了每个部门薪水最高的员工。这种类型的查询对于找出每个分组内特定条件下的记录非常有用。
通过这些查询，我们可以看到如何结合使用 MySQL 的各种功能来进行复杂的数据分析。聚合函数和 GROUP BY 子句能够帮助我们汇总数据，而子查询和连接操作则允许我们将这些汇总结果与其他表或查询结果结合起来，进行进一步的分析和筛选。
显示每个部门的信息（部门号、编号、地址）和人员数量
sql
mysql> select count(*) from emp;
+----------+
| count(*) |
+----------+
| 14 |
+----------+
1 row in set (0.00 sec)
mysql> select count(*) from emp group by deptno;
+----------+
| count(*) |
+----------+
| 3 |
| 5 |
| 6 |
+----------+
3 rows in set (0.00 sec)
mysql> select deptno,count(*) from emp group by deptno;
+--------+----------+
| deptno | count(*) |
+--------+----------+
| 10 | 3 |
| 20 | 5 |
| 30 | 6 |
+--------+----------+
3 rows in set (0.00 sec)
mysql> select *from dept t1,(select deptno,count(*) from emp group by deptno) t2;
+--------+------------+----------+--------+----------+
| deptno | dname | loc | deptno | count(*) |
+--------+------------+----------+--------+----------+
| 10 | ACCOUNTING | NEW YORK | 10 | 3 |
| 20 | RESEARCH | DALLAS | 10 | 3 |
| 30 | SALES | CHICAGO | 10 | 3 |
| 40 | OPERATIONS | BOSTON | 10 | 3 |
| 10 | ACCOUNTING | NEW YORK | 20 | 5 |
| 20 | RESEARCH | DALLAS | 20 | 5 |
| 30 | SALES | CHICAGO | 20 | 5 |
| 40 | OPERATIONS | BOSTON | 20 | 5 |
| 10 | ACCOUNTING | NEW YORK | 30 | 6 |
| 20 | RESEARCH | DALLAS | 30 | 6 |
| 30 | SALES | CHICAGO | 30 | 6 |
| 40 | OPERATIONS | BOSTON | 30 | 6 |
+--------+------------+----------+--------+----------+
12 rows in set (0.00 sec)
mysql> select *from dept t1,(select deptno,count(*) from emp group by deptno) t2 where t1.deptno=t2.deptno;
+--------+------------+----------+--------+----------+
| deptno | dname | loc | deptno | count(*) |
+--------+------------+----------+--------+----------+
| 10 | ACCOUNTING | NEW YORK | 10 | 3 |
| 20 | RESEARCH | DALLAS | 20 | 5 |
| 30 | SALES | CHICAGO | 30 | 6 |
+--------+------------+----------+--------+----------+
3 rows in set (0.00 sec)
mysql> select t1.dname,t1.loc,t2.dept_num ,t1.deptno from dept t1,(select deptno,count(*) dept_num from emp group by deptno) t2 where t1
1.deptno=t2.deptno;
+------------+----------+----------+--------+
| dname | loc | dept_num | deptno |
+------------+----------+----------+--------+
| ACCOUNTING | NEW YORK | 3 | 10 |
| RESEARCH | DALLAS | 5 | 20 |
| SALES | CHICAGO | 6 | 30 |
+------------+----------+----------+--------+
3 rows in set (0.00 sec)
mysql>
mysql>
mysql>
mysql>
mysql> select *from emp,dept;
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno | deptno | dname | loc |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
| 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 10 | ACCOUNTING | NEW YORK |
| 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 30 | SALES | CHICAGO |
| 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 40 | OPERATIONS | BOSTON |
| 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 10 | ACCOUNTING | NEW YORK |
| 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 20 | RESEARCH | DALLAS |
| 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 30 | SALES | CHICAGO |
| 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 40 | OPERATIONS | BOSTON |
| 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 10 | ACCOUNTING | NEW YORK |
| 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 20 | RESEARCH | DALLAS |
| 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 30 | SALES | CHICAGO |
| 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 40 | OPERATIONS | BOSTON |
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 10 | ACCOUNTING | NEW YORK |
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 30 | SALES | CHICAGO |
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 40 | OPERATIONS | BOSTON |
| 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 10 | ACCOUNTING | NEW YORK |
| 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 20 | RESEARCH | DALLAS |
| 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 30 | SALES | CHICAGO |
| 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 40 | OPERATIONS | BOSTON |
| 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 10 | ACCOUNTING | NEW YORK |
| 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 20 | RESEARCH | DALLAS |
| 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 30 | SALES | CHICAGO |
| 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 40 | OPERATIONS | BOSTON |
| 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK |
| 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 20 | RESEARCH | DALLAS |
| 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 30 | SALES | CHICAGO |
| 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 40 | OPERATIONS | BOSTON |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 10 | ACCOUNTING | NEW YORK |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 30 | SALES | CHICAGO |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 40 | OPERATIONS | BOSTON |
| 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK |
| 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 20 | RESEARCH | DALLAS |
| 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 30 | SALES | CHICAGO |
| 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 40 | OPERATIONS | BOSTON |
| 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 10 | ACCOUNTING | NEW YORK |
| 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 20 | RESEARCH | DALLAS |
| 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 30 | SALES | CHICAGO |
| 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 40 | OPERATIONS | BOSTON |
| 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 10 | ACCOUNTING | NEW YORK |
| 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 30 | SALES | CHICAGO |
| 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 40 | OPERATIONS | BOSTON |
| 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 10 | ACCOUNTING | NEW YORK |
| 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 20 | RESEARCH | DALLAS |
| 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 30 | SALES | CHICAGO |
| 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 40 | OPERATIONS | BOSTON |
| 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 10 | ACCOUNTING | NEW YORK |
| 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 30 | SALES | CHICAGO |
| 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 40 | OPERATIONS | BOSTON |
| 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK |
| 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 20 | RESEARCH | DALLAS |
| 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 30 | SALES | CHICAGO |
| 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 40 | OPERATIONS | BOSTON |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
56 rows in set (0.00 sec)
mysql> select *from emp,dept where emp.deptno =dept.deptno;
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno | deptno | dname | loc |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
| 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 30 | SALES | CHICAGO |
| 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 30 | SALES | CHICAGO |
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 30 | SALES | CHICAGO |
| 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 30 | SALES | CHICAGO |
| 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK |
| 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 30 | SALES | CHICAGO |
| 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 30 | SALES | CHICAGO |
| 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
14 rows in set (0.00 sec)
mysql> select *from emp,dept where emp.deptno =dept.deptno group by emp.deptno;
ERROR 1055 (42000): Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'scott.emp.empno' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by
mysql> select count(*) from emp,dept where emp.deptno =dept.deptno group by emp.deptno;
+----------+
| count(*) |
+----------+
| 3 |
| 5 |
| 6 |
+----------+
3 rows in set (0.00 sec)
mysql> select deptno, count(*) from emp,dept where emp.deptno =dept.deptno group by emp.deptno;
ERROR 1052 (23000): Column 'deptno' in field list is ambiguous
mysql> select emp.deptno, count(*) from emp,dept where emp.deptno =dept.deptno group by emp.deptno;
+--------+----------+
| deptno | count(*) |
+--------+----------+
| 10 | 3 |
| 20 | 5 |
| 30 | 6 |
+--------+----------+
3 rows in set (0.00 sec)
mysql> select dept.dname,emp.deptno, count(*) from emp,dept where emp.deptno =dept.deptno group by emp.deptno;
ERROR 1055 (42000): Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'scott.dept.dname' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by
mysql> select dept.dname,emp.deptno, count(*) from emp,dept where emp.deptno =dept.deptno group by emp.deptno,dept.dname ;
+------------+--------+----------+
| dname | deptno | count(*) |
+------------+--------+----------+
| ACCOUNTING | 10 | 3 |
| RESEARCH | 20 | 5 |
| SALES | 30 | 6 |
+------------+--------+----------+
3 rows in set (0.00 sec)
mysql> select dept.dname,emp.deptno, count(*),dept.loc from emp,dept where emp.deptno =dept.deptno group by emp.deptno,dept.dname ;
ERROR 1055 (42000): Expression #4 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'scott.dept.loc' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by
mysql> select dept.dname,emp.deptno, count(*),dept.loc from emp,dept where emp.deptno =dept.deptno group by emp.deptno,dept.dname,dept.loc ;
+------------+--------+----------+----------+
| dname | deptno | count(*) | loc |
+------------+--------+----------+----------+
| ACCOUNTING | 10 | 3 | NEW YORK |
| RESEARCH | 20 | 5 | DALLAS |
| SALES | 30 | 6 | CHICAGO |
+------------+--------+----------+----------+
3 rows in set (0.00 sec)
mysql>
mysql>
mysql>
mysql> select dept.dname,emp.deptno, count(*) total,dept.loc from emp,dept where emp.deptno =dept.deptno group by emp.deptno,dept.dname,,dept.loc ;
+------------+--------+-------+----------+
| dname | deptno | total | loc |
+------------+--------+-------+----------+
| ACCOUNTING | 10 | 3 | NEW YORK |
| RESEARCH | 20 | 5 | DALLAS |
| SALES | 30 | 6 | CHICAGO |
+------------+--------+-------+----------+
3 rows in set (0.00 sec)
mysql> select dept.dname,emp.deptno, count(*) total,dept.loc from emp,dept where emp.deptno =dept.deptno group by emp.deptno,dept.dname,dept.loc order by total desc ;
+------------+--------+-------+----------+
| dname | deptno | total | loc |
+------------+--------+-------+----------+
| SALES | 30 | 6 | CHICAGO |
| RESEARCH | 20 | 5 | DALLAS |
| ACCOUNTING | 10 | 3 | NEW YORK |
+------------+--------+-------+----------+
3 rows in set (0.00 sec)
mysql> 这一系列的 MySQL 查询演示了如何使用 COUNT() 函数结合 GROUP BY 子句来统计每个部门的员工数量，并通过连接 emp 和 dept 表来获取更丰富的部门信息。
首先，查询了 emp 表中的总员工数。
然后，通过对 emp 表使用 GROUP BY deptno 子句，查询了每个部门的员工数量。
通过将 deptno 和员工数量同时选择，明确了每个部门编号及其对应的员工数量。
接着，尝试将 emp 表和 dept 表进行笛卡尔积查询，然后限制了结果集，只显示那些部门编号匹配的记录，从而提供了每个部门的详细信息及其员工数量。
通过对连接查询的结果应用 GROUP BY 子句，错误地尝试了将非聚合列包含在选择列表中，但没有将它们加入 GROUP BY 子句，这违反了启用 only_full_group_by SQL 模式时的要求。
修正了上述问题，通过在 GROUP BY 子句中明确指定所有非聚合列，成功地获取了每个部门的名称、编号、员工数量和位置。
最后，通过修改选择列表和 GROUP BY 子句，按员工总数降序排列了结果，从而可以快速识别员工数量最多和最少的部门。
小结论
解决多表问题的本质：想办法将多表转化成为单表，所以mysql中，所有select的问题全部都可以转成单表问题！！
结尾
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