Question

I’m using MySQL 5.1

Here’s what I am trying to do:

SELECT name, gender, age, partners.rate_M, partners.rate_F
  FROM users INNER JOIN partners ON users.id = partners.M_id

Currently what I have is:

SELECT name, gender, age, partners.rate_M, partners.rate_F
  FROM users INNER JOIN partners
    ON (users.id = partners.M_id OR users.id = partners.F_id)

Which slows my system.

M in M_id is the gender, so how can I make the M dynamic?

users.gender + '_id' = M_id or F_id

Answer 1

You can’t do it directly, so you have to use an indirect mechanism.

One simple option is a UNION:

SELECT u.name, u.gender, u.age, p.rate_M, p.rate_F
  FROM users AS u INNER JOIN partners AS p ON u.id = p.M_id AND u.gender = 'M'
UNION
SELECT u.name, u.gender, u.age, p.rate_M, p.rate_F
  FROM users AS u INNER JOIN partners AS p ON u.id = p.F_id AND u.gender = 'F';