Question

我是否可以通过某种方式编写此查询，以获取与我的category_id相比较的子查询结果。

SELECT category_id,
count(id) as TOTAL COUNT, 
(select count(*) from products where product_path LIKE '%Electronics%'
 and category_id = category_id ) as ELECTRONIC COUNT
FROM products
WHERE product_path LIKE '%Products%'
GROUP BY category_id

我想以下面的方式结果：

"category_id"   "TOTAL COUNT"   "ELECTRONIC COUNT"
   "173"              "1"               "243"
    "42"              "1"               "243"
   "211"              "41"              "243"
   "162"              "10"              "243"
   "172"              "139"             "243"
   "116"              "54"              "243"
    "10"              "3"               "243"

我希望电子计数取决于类别。即，第一行应该是category_id = 173，第二行应该是category_id = 42，第三行应该是category_id = 211等等。

Answer 1

要使相关子查询与同一个表一起使用，您必须使用table aliases：

SELECT category_id
      ,count(*) AS total_count  -- unquoted column alias with space is wrong, too
      ,(SELECT count(*)
        FROM   products AS p1
        WHERE  product_path LIKE '%Electronics%'
        AND    p1.category_id = p.category_id
       ) AS electronic_count
FROM   products AS p
WHERE  product_path LIKE '%Products%'
GROUP  BY category_id;

假设id是主键，因此NOT NULL。count(*)。然后SELECT category_id ,count(*) AS total_count -- keyword AS is needed for column alias ,count(product_path LIKE '%Electronics%' OR NULL) AS electronic_count FROM products p -- keyword AS is just noise for table alias WHERE product_path LIKE '%Products%' GROUP BY category_id;做得更好。

但这可以进一步简化到：

count()

快得多。
OR NULL仅计算非空值。添加FALSE我将NULL转换为product_path LIKE '%Electronics%'。因此，只有那些行计数，TRUE评估为{{1}}。

将子查询添加到依赖于第一列的SELECT语句？

1 个答案: