MySQL where clause targeting dependent subquery

Question

I work with entity-attribute-value database model.

My product have many attribute_varchars and attribute_varchars have one attribute. An attribute has many attribute_varchars and an attribute varchar has one product. Same logic apply to attribute_decimals and attribute_texts.

Anyway, i have the following query and i would like to filter the result using a where clause

SELECT 
    products.id,
    (select value from attribute_texts    where product_id = products.id and attribute_id = 1) 
        as description,
    (select value from attribute_varchars where product_id = products.id and attribute_id = 2) 
        as model,
    (select value from attribute_decimals where product_id = products.id and attribute_id = 9) 
        as rate,
    (select value from attribute_varchars where product_id = products.id and attribute_id = 20) 
        as batch
FROM products
WHERE products.status_id <> 5

I would like to add a where rate > 5

I tried but I get the following error : Unknown column 'rate' in 'where clause'. I tried adding an alias to the value and to the table of the value but nothing seems to work.

Answer 1

In MySQL, you can do:

having rate > 5

MySQL has extended the having clause so it can work without a group by. Although questionable as a feature, it does allow you to reference aliases in the select clause without using a subquery.

Answer 2

With generated column like that, the best way is to made your query as a subquery, and do your filtering on the upper level like that:

SELECT *
FROM
(
SELECT 
    products.id,
    (select value from attribute_texts    where product_id = products.id and attribute_id = 1) 
        as description,
    (select value from attribute_varchars where product_id = products.id and attribute_id = 2) 
        as model,
    (select value from attribute_decimals where product_id = products.id and attribute_id = 9) 
        as rate,
    (select value from attribute_varchars where product_id = products.id and attribute_id = 20) 
        as batch

FROM products
WHERE products.status_id <> 5
) as sub
WHERE sub.Rate > 5

Answer 3

Use join clauses rather than subqueries for the attribute tables:

SELECT 
    p.id,
    a_description.value AS description,
    a_model.value       AS model,
    a_rate.value        AS rate,
    a_batch.value       AS batch
FROM products p
INNER JOIN attribute_texts a_description 
    ON  a_description.product_id = p.id
    AND a_description.attribute_id = 1
INNER JOIN attribute_varchars a_model 
    ON  a_model.product_id = p.id
    AND a_model.attribute_id = 2
INNER JOIN attribute_decimals a_rate 
    ON  a_rate.product_id = p.id
    AND a_rate.attribute_id = 9
INNER JOIN attribute_varchars a_batch
    ON  a_batch.product_id = p.id
    AND a_batch.attribute_id = 20
WHERE products.status_id <> 5
  AND a_rate.value > 5