Using the assigned name of a field with 'AS' within a JOIN - mysql

Question

I have this code..
SELECT u.*, COALESCE(c2.name,u.state) AS real_state, c.name AS real_country, bc.alpha3 AS country_iso FROM gimko_v2.users AS u
INNER JOIN gimko_v2.countries AS c ON u.country=c.id
LEFT JOIN gimko_v2.countries AS c2 ON u.state=c2.id
LEFT JOIN gimko_blesta.countries AS bc ON c.country_iso_code=bc.alpha2
LEFT JOIN gimko_blesta.states AS bs ON bc.alpha2=bs.country_alpha2 AND real_state=bs.name
WHERE u.blesta_id=0;
The problem likes in this area:
COALESCE(c2.name,u.state) AS real_state
and
LEFT JOIN gimko_blesta.states AS bs ON bc.alpha2=bs.country_alpha2 AND real_state=bs.name
I am getting this error:
Error Code: 1054
Unknown column 'real_state' in 'on clause'
How can I use the result of COALESCE(c2.name,u.state) in a JOIN?
Thanks!

Answer 1

Does gimko_blesta.states even need to be in the source tables? None of its columns are in the select list/criteria, and it's joined in with a LEFT JOIN, so doesn't seem like it's necessary.
In any case, you would need to make it a derived table first, and join gimko_blesta.states to the derived table, or just use the expression in the JOIN, like COALESCE(c2.name,u.state) = bs.name, below:
SELECT u.*, COALESCE(c2.name,u.state) AS real_state, c.name AS real_country, bc.alpha3 AS country_iso
FROM gimko_v2.users AS u
INNER JOIN gimko_v2.countries AS c ON u.country=c.id
LEFT JOIN gimko_v2.countries AS c2 ON u.state=c2.id
LEFT JOIN gimko_blesta.countries AS bc ON c.country_iso_code=bc.alpha2
LEFT JOIN gimko_blesta.states AS bs ON bc.alpha2=bs.country_alpha2 AND COALESCE(c2.name,u.state) = bs.name
WHERE u.blesta_id=0;

Answer 2

You can't use virtual aliases at the same level of your query you may have to repeat the whole expression or use a sub select to access in your parent query
SELECT * FROM (
SELECT u.*,
COALESCE(c2.name,u.state) AS real_state,
c.name AS real_country,
bc.alpha3 AS country_iso ,
bc.alpha2
FROM gimko_v2.users AS u
INNER JOIN gimko_v2.countries AS c ON u.country=c.id
LEFT JOIN gimko_v2.countries AS c2 ON u.state=c2.id
LEFT JOIN gimko_blesta.countries AS bc ON c.country_iso_code=bc.alpha2
) t
LEFT JOIN gimko_blesta.states AS bs ON t.alpha2=bs.country_alpha2 AND t.real_state=bs.name
WHERE t.blesta_id=0;
Also in sub-select you have to select all the needed columns that will be used in outer join for parent query like in your case i have selected bc.alpha2 in inner query because to join with states this column is needed