MySQL未知列使用子查询

Question

I receive an error when i execute this query:

SELECT 
    (SELECT count(cp_projeto_view.id) FROM cp_projeto_view WHERE cp_projeto_view.id_projeto = cp_projeto.id AND cp_projeto_view.id_pessoa = 467 LIMIT 1) AS qtde_visualizacoes
FROM cp_projeto
WHERE qtde_visualizacoes = 0

The error is: #1054 - Unknown column 'qtde_visualizacoes' in 'where clause'

Why qtde_visualizacoes does not exists?

Thank you very much!

Answer 1

You cannot use a column alias in a where clause (unless you use a subquery). MySQL has an extension where you can use having instead:

SELECT (SELECT count(cp_projeto_view.id)
        FROM cp_projeto_view
        WHERE cp_projeto_view.id_projeto = cp_projeto.id AND cp_projeto_view.id_pessoa = 467
        LIMIT 1
       ) AS qtde_visualizacoes
FROM cp_projeto
HAVING qtde_visualizacoes = 0

EDIT:

The query that you probably want is more like:

select p.*
from cp_projecto p
where not exists (select 1 from cp_projeto_view pv where pv.id_projeto = p.id and pv.id_pessoa = 467)

This will return all cp_projecto s that have no matching rows in cp_projeto_view . Your original query would only return rows with a single column of 0 s, which doesn't make much sense. If you want the count, then do:

select count(*) as cnt
from cp_projecto p
where not exists (select 1 from cp_projeto_view pv where pv.id_projeto = p.id and pv.id_pessoa = 467)

And, for performance, create an index on cp_projeto_view(projeto, id_pessoa) .