如何通过来自另一个表但不同类型的多个连接记录来过滤数据库表？

Question

I have a products table and corresponding ratings table which contains a foreign key product_id , grade(int) and type which is an enum accepting values robustness and price_quality_ratio

The grades accept values from 1 to 10. So for example, how would the query look like, if I wanted to filter the products where minimum grade for robustness would be 7 and minimum grade for price_quality_ratio would be 8?

Answer 1

You can join twice, once per rating. The inner join s eliminate the products that fail any rating criteria,

select p.*
from products p
inner join rating r1 
    on r1.product_id = p.product_id
    and r1.type = 'robustness'
    and r1.rating >= 7
inner join rating r2 
    on r2.product_id = p.product_id
    and r2.type = 'price_quality_ratio'
    and r2.rating >= 8

Another option is to use do conditional aggregation. This requires only one join , then a group by ; the rating criteria are checked in the having clause.

select p.product_id, p.product_name
from products p
inner join rating r 
    on r.product_id = p.product_id
    and r.type in ('robustness', 'price_quality_ratio')
group by p.product_id, p.product_name
having 
    min(case when r.type = 'robustness' then r.rating end) >= 7
    and min(case when r.type = 'price_quality_ratio then r.rating end) >= 8

Answer 2

The JOIN proposed by @GMB would've been my first suggestion as well. If that gets too complicated with having to maintain too many rX.rating s, you can also use a nested query:

SELECT *
FROM (
  SELECT p.*, r1.rating as robustness, r2.rating as price_quality_ratio
  FROM products p
  JOIN rating r1 ON (r1.product_id = p.product_id AND r1.type = 'robustness')
  JOIN rating r2 ON (r2.product_id = p.product_id AND r2.type = 'price_quality_ratio')
) AS tmp
WHERE robustness >= 7
  AND price_quality_ratio >= 8
-- ORDER BY (price_quality_ratio DESC, robustness DESC) -- etc