问题
A MySQL database contains two tables with a one-to-many relationship: One user can have many settings:
Users:
id username password
--------------------------
1 Bob 123
2 Alice abc
...
Settings:
id user_id key value
-----------------------------
1 1 color blue // Bobs settings...
2 1 theme xy
3 1 size 5
4 2 size 5 // Alices settings...
Problem: How to find all users with color == blue AND size == 5?
Using a LEFT JOIN it is no problem to find users with one property:
SELECT users.id FROM users LEFT JOIN settings ON users.id = settings.user_id WHERE settings.key = 'color' AND settings.value = 'blue'
However, this does not work when searching for two settings at a time?
Is it possible to solve this with a single statement? What is the most efficient way to query this data?
回答1:
One method uses aggregation and having:
select s.user_id
from settings s
where (key, value) in ( ('color', 'blue'), ('size', '5') )
group by s.user_id
having count(*) = 2;
This assumes that there are no duplicate settings (if so, you would need to use count(distinct)).
来源：https://stackoverflow.com/questions/51153040/sql-one-to-many-relationship-how-to-select-rows-depending-on-multiple-to-many