mysql(四)-增删改查
avg() 返回指定列的平均值 max() 返回指定列的最大值min() 返回指定列的最小值
count() 返回指定列中非null值的个数
sum() 返回指定列的所有值之和
select count(Language) from countrylanguage group by CountryCode limit 10;
+-----------------+
| count(Language) |
+-----------------+
| 4 |
| 5 |
| 9 |
| 1 |
| 3 |
| 4 |
HAVING: 对分组聚合运算后的结果指定过滤条件
select count(Language) from countrylanguage group by CountryCode having count(Language) > 5 limit 10;
+-----------------+
| count(Language) |
+-----------------+
| 9 |
| 8 |
| 8 |
| 6 |
| 7 |
| 6 |
| 7 |
| 6 |
| 12 |
| 12 |
+-----------------+
查询students表，以性别为分组，求出分组后的年龄之和。
select gender,sum(age) from students group by gender;
查询students表，以classid分组，显示平均年龄大于25的classid。
select> 查询students表，以性别字段gender分组，显示各组中年龄大于19的学员的年龄的总和。
select sum(age) from students where age > 19 group by gender;
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