0

我的查询工作正常，但它有 2 个相同的子查询

SELECT *, 
    (SELECT count(O.id) FROM `offer` O WHERE O.product_id = P.id) AS poffers 
FROM `product` P 
JOIN product_section PS ON (PS.product_id = P.id AND PS.section_id IN (14)) 
WHERE P.deleted is NULL 
    AND (SELECT count(O.id) FROM `offer` O WHERE O.product_id = P.id) > 0

我尝试使用 JOIN 进行优化，但新查询返回空结果

SELECT *, OJ.pcount AS poffers
FROM `product` P 
JOIN product_section PS ON PS.product_id = P.id AND PS.section_id IN (14)
JOIN (SELECT count(O.id) AS pcount, O.product_id FROM `offer` O ) 
    AS OJ ON OJ.product_id = P.id
WHERE P.deleted is NULL AND OJ.pcount > 0

比我尝试变量，但我认为我用错了，查询返回空结果

SELECT *,  
    @o := (SELECT count(O.id) FROM `offer` O WHERE O.product_id = P.id) 
    AS poffers 
FROM `product` P 
JOIN product_section PS ON (PS.product_id = P.id AND PS.section_id IN (14)) 
WHERE P.deleted is NULL 
    AND @o > 0
4

2 回答 2

1

试一试（未测试）

SELECT P.*,PS.*,count(O.id) AS oCount
FROM `product` P
INNER JOIN `product_section` PS ON (PS.product_id = P.id)
INNER JOIN `offer` O ON (O.product_id = P.id) 
WHERE P.deleted is NULL AND PS.section_id =14
GROUP BY O.product_id
HAVING oCount> 0

更优化

SELECT P.*,PS.*,count(O.id) AS oCount
FROM `product` P
INNER JOIN (`product_section` PS,`offer` O) ON 
(PS.product_id = P.id AND O.product_id = P.id) 
WHERE P.deleted is NULL AND PS.section_id =14
GROUP BY O.product_id
HAVING oCount> 0

尖端：

  • 不要使用*，而是只写那些你想检索的列名
  • ON对使用 with &WHERE条件的所有列进行索引

Reference

于 2012-07-26T15:38:21.813 回答
1

避免依赖子查询。相反，有一个查询将产品与报价计数相关联。您可以在多个地方使用此查询。它很可能会临时存储在内存中，避免重复计算，但数据库引擎也可能会做更聪明的事情。

SELECT *, O.cnt AS poffers
FROM product P
JOIN product_section PS ON (PS.product_id = P.id AND PS.section_id IN (14))
JOIN (SELECT product_id, count(id) AS cnt
      FROM offer
      GROUP BY product_id
     ) O ON (O.product_id = P.id)
WHERE P.deleted is NULL AND O.cnt > 0

如果 P 和 PS 之间的关系是一对一的，那么您甚至可以在没有任何子查询的情况下工作：

SELECT *, COUNT(O.id) AS poffers
FROM product P
JOIN product_section PS ON (PS.product_id = P.id AND PS.section_id IN (14))
JOIN offer O ON (O.product_id = P.id)
WHERE P.deleted is NULL
GROUP BY P.id

您还应该考虑是否有充分的理由计算特定列。在大多数情况下，COUNT(*)会比COUNT(col). 使用后者的唯一原因是如果您明确想要NULL从计数中排除值。

于 2012-07-26T16:13:17.417 回答