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[英]In MySQL 5.7 how would I select all parent table rows with less than 2 related child table rows
[英]Mysql - Select rows which have less than x “child rows”
我有一张这样的桌子:
planet_id | planet_name
1 | Test planet
2 | Test planet 2
我有第二张桌子
area_id | area_name | planet_id
1 | test_area | 1
我需要一个选择查询来仅选择面积小于5的行星。 我怎么做?
您可以将按区域划分的行星分组并按行星分组。 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
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