php - mysql select/delete using join over four tables -

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i have 4 tables (in [] columns):

users [id]

products [id]

productratings [id,value,user,product]

comments [id,product,user]

i select/and delete productratings there no associated comment same user product. is, if user has rated product did not comment, rating should deleted.

i believe achieve using 2 queries, first:

select user, product productratings

and each row:

    select count(*) comments product=productratings.product , user=productratings.user

and like

    if $queryabove==0 : delete productratings id=productratings.id

i solve via join , learn more example rather dig through join tutorials.

you need productratings , comments table - following works:

delete pr productratings pr left join comments c   on pr.userid = c.userid   , pr.productid = c.productid c.productid null

and there's demo here: http://sqlfiddle.com/#!9/89575/1


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