[英]Mysql - Select rows which have less than x “child rows”
I got a table like this: 我有一张这样的桌子:
planet_id | planet_name
1 | Test planet
2 | Test planet 2
I got a second table like this 我有第二张桌子
area_id | area_name | planet_id
1 | test_area | 1
I need a select query to only select planets which have less than 5 areas. 我需要一个选择查询来仅选择面积小于5的行星。 How do I do that?
我怎么做?
you can group by planets joined with areas and group by the planets. 您可以将按区域划分的行星分组并按行星分组。 you can then use having to filter
然后您可以使用过滤
select one.planet_id, one.planet_name
from first_table one
join second_table two
on (one.planet_id = two.planet_id)
group by one.planet_id, one.planet_id
having count(two.area_id) < 5
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.