分组过滤
对分组结果进行过滤，必须使用HAVING子句，不能使用WHERE字句，WHERE字句是针对行的过滤。
- 单独使用HAVING字句
语法：
select column,group_function(column)
from table
[group by group_by_expression]
[having group_condition]
[order by column]
执行顺序：from>group by>having>select>order by
- HAVING字句与WHERE字句结合
语法：
select column,group_function(column)
from table
[where condition]
[group by group_by_expression]
[having group_condition]
[order by column]
执行顺序：from>where>group by>having>select>order by
例子：
示例Table：
1.JPG
- 分组GROUP BY
SELECT subject, COUNT(*) AS subject_count FROM score_table GROUP BY subject;
2.JPG
- WHERE + GROUP BY
SELECT subject, COUNT(*) AS subject_count FROM score_table WHERE score>80 GROUP BY subject;
3.JPG
- HAVING + GROUP BY
SELECT subject, COUNT(*) AS subject_count FROM score_table GROUP BY subject HAVING subject_count>2;
2.JPG
- WHERE + GROUP BY +HAVING
SELECT subject, COUNT(*) AS subject_count FROM score_table WHERE score>80 GROUP BY subject HAVING subject_count>=2;
5.JPG
