Question

Hello I need help in writing a query.

Below is my DB structure

id  name    key    value
1   book1   key1    abc
2   book2   key2    aaa
3   book3   key5    abc
4   book4   key3    abc
5   book5   key2    aaa
6   book6   key2    aab
7   book7   key1    abc
8   book8   key2    abc
9   book9   key1    abc
10  book10  key1    abc

And I need those books whose key has combination key1 with value abc and key2 with value aaa.

SO it should return

1   book1   key1    abc
2   book2   key2    aaa
5   book5   key2    aaa
7   book7   key1    abc
9   book9   key1    abc
10  book10  key1    abc

book having key2 and value aab wont return.

P.s This is just an example structure I have made to demonstrate my need. Actually I am working on wordpress post meta table.

Answer 1

I think that according to your design at some point in time you’ll have several key + value pairs per book. This means that you might wish to select books where several specific keys has some particular values.

This can be achieved with the following SQL (for the given example):

SELECT t.id,t.name,k.key1,k.key2
  FROM test t
  JOIN (SELECT t.id,k1.value AS key1,k2.value AS key2
          FROM test t
          LEFT JOIN test k1 ON t.id=k1.id AND k1.`key`='key1'
          LEFT JOIN test k2 ON t.id=k2.id AND k2.`key`='key2'
  ) k ON t.id = k.id
WHERE k.key1='abc' OR k.key2 = 'aaa';

The inner part will create a pivoted table with keys in subject, while the outer part will perform the result filtering. You can play around with this setup here.

Though, for such design, I would recommend splitting the table in subject into 2 separate ones: books and params. You can observe such setup here.
Note, that if you’ll change filter condition to be k.key1='abc' AND k.key2 = 'aaa', then you’ll have only book10 matching.

This example is quite simple, but it gives the idea.