Usage of mysql having

一、 having的用法

having字句可以让我们筛选成组后的各种数据,where字句在聚合前先筛选记录,也就是说作用在group by和having字句前.而 having子句在聚合后对组记录进行筛选.

二、SQL实例

1. 显示每个地区的总人口数和总面积．

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

先以region把返回记录分成多个组,这就是GROUP BY的字面含义.分完组后,然后用聚合函数对每组中的不同字段（一或多条记录）作运算.

2. 显示每个地区的总人口数和总面积．仅显示那些面积超过1000000的地区.

//在这里,我们不能用where来筛选超过1000000的地区,因为表中不存在这样一条记录.相反,having子句可以让我们筛选成组后的各组数据
SELECT region, SUM(population), SUM(area) FROM bbc GROUP BY region HAVING SUM(area)>1000000

3. mysql判断某个字段的长度：

select home_page from aaa表 where char_length(trim(home_page))<10 and char_length(trim(home_page))>1;

三、mysql中的where和having子句的区别

1. mysql中的where和having子句The function of filtering records can be realized,But there are some differences in their usage.

用group by和havingClause union to find distinct records,sql如下:

select uid,email,count(*) as ct from `edm_user081217` GROUP BY email

然后看这个,就容易理解了

//先用group by 对email进行分组,在用havingto filter greater than1的,This finds out duplicate records
select uid,email,count(*) as ct from `edm_user081217` GROUP BY email HAVING ct > 1

2. 以下是having和where的区别：

Select city FROM weather WHERE temp_lo = (SELECT max(temp_lo) FROM weather);

• 作用的对象不同.

• WHERE 子句作用于表和视图
• HAVING 子句作用于组.

WHERE 在分组和聚集计算之前选取输入行（因此,它控制哪些行进入聚集计算）, 而 HAVING 在分组和聚集之后选取分组的行.因此,WHERE 子句不能包含聚集函数; Because it doesn't make sense to try to use an aggregate function to determine which rows are input to the aggregate operation. 相反,HAVING 子句总是包含聚集函数.（严格说来,You can write without using aggregation HAVING 子句, But doing so is in vain.The same conditions can be used more effectively WHERE 阶段.）
在前面的例子里,我们可以在 WHERE City name restrictions apply,Because it doesn't require aggregation. 这样比在 HAVING It is more efficient to increase the limit here,Because we avoid for those that don't pass WHERE Examine the rows for grouping and aggregation calculations
综上所述：

• having一般跟在group by之后,Performs part of the record group selection to work.
• whereIt is the execution of all data to work.
• 再者havingAggregate functions can be used,如having sum(qty)>1000