Question

I have m:n relationship between users and tags. One user can have m tags, and one tag can belong to n users. Tables look something like this:

USER:
ID
USER_NAME

USER_HAS_TAG:
USER_ID
TAG_ID

TAG:
ID
TAG_NAME

Let’s say that I need to select all users, who have tags “apple”, “orange” AND “banana”. What would be the most effective way to accomplish this using SQL (MySQL DB)?

Answer 1

In addition to the other good answers, it’s also possible to check the condition in a WHERE clause:

select *
from user u
where 3 = (
    select count(distinct t.id)
    from user_has_tag uht
    inner join tag t on t.id = uht.tag_id
    where t.name in ('apple', 'orange', 'banana') 
    and uht.user_id = u.userid
)

The count(distinct ...) makes sure a tag is counted only once, even if the user has multiple ‘banana’ tags.

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