浅谈sql语句中GROUP BY 和 HAVING的使用方法

SELECT region, SUM(population), SUM(area)
FROM bbc
GROUP BY region

SELECT region, SUM(population), SUM(area)

FROM bbc8 F4 w2 v( P- f
GROUP BY region
HAVING SUM(area)>1000000#

select c.name, count(order_number) as count from orders o,customer c where c.id=o.customer_id group by customer_id;

+--------+-------+
| name | count |
+--------+-------+
| d | 9 |
| cc | 6 |
| 菩提子 | 1 |
| cccccc | 2 |
+--------+-------+

select c.name, count(order_number) as count from orders o,customer c where c.id=o.customer_id group by customer_id having count(order_number)>5;

+------+-------+
| name | count |
+------+-------+
| d | 9 |
| cc | 6 |
+------+-------+

SQL> select * from sc;

 SNO PNO GRADE
---------- ----- ----------
  1 YW  95
  1 SX  98
  1 YY  90
  2 YW  89
  2 SX  91
  2 YY  92
  3 YW  85
  3 SX  88
  3 YY  96
  4 YW  95
  4 SX  89

 SNO PNO GRADE
---------- ----- ----------
  4 YY  88

SQL> select sno,pno,grade from sc where grade>=90;

 SNO PNO GRADE
---------- ----- ----------
  1 YW  95
  1 SX  98
  1 YY  90
  2 SX  91
  2 YY  92
  3 YY  96
  4 YW  95

//进行分组显示，并且按照where条件之后计数

SQL> select sno,count(*) from sc where grade>=90 group by sno;

 SNO COUNT(*)
---------- ----------
  1  3
  2  2
  4  1
  3  1

//进行分组显示，并且按照where条件之后计数，在根据having子句筛选分组

SQL> select sno,count(*) from sc where grade>=90 group by sno having count(*)>=2；

 SNO COUNT(*)
---------- ----------
  1  3
  2  2

//实际上，这个查询先把语文大于95分的学生号提取出来，之后求平均值，分组显示后根据having语句选出平均成绩大于90的

SQL> select sno,avg(grade) from sc where SNO IN (SELECT SNO FROM SC WHERE GRADE>=95 AND PNO='YW') group by sno having avg(grade)>=90;

 SNO AVG(GRADE)
---------- ----------
  1 94.3333333
  4 90.6666667

//having子句中可进行比较和子查询

SQL> select sno,avg(grade) from sc
 group by sno
 having avg(grade) > (select avg(grade) from sc where sno=3);