Question

我有一张表Content

内容（id，contentText，contentDate，idUser→User，contentType）

和表VoteQuestion

VoteQuestion （idUser→用户，idQuestion→内容，isUp）

我想选择前30个最近的问题。

SELECT * 
FROM "Content" 
WHERE "type" = 'QUESTION'
ORDER BY "contentDate"
LIMIT 30

但是，我想连接到另一个包含与此问题相关的信息的列，因此我不需要再次为每个返回的问题查询数据库。

例如，我想计算每个问题的投票数，并将其全部返回到同一行。

示例：

| id | contentText | contentDate | idUser | contentType | votes |
-----------------------------------------------------------------
| 2  |   'abc'     | '2013-03-25'|  192   |  'QUESTION' |  10   |

我尝试了以下查询：

WITH question AS
(SELECT * 
FROM "Content" 
WHERE "type" = 'QUESTION'
ORDER BY "contentDate"
LIMIT 30
)

SELECT COUNT("VoteQuestion"."idUser") 
FROM "VoteQuestion", question
WHERE "idQuestion" = question."id"
GROUP BY question."id";

但这不会返回投票数= 0的问题（只返回16个问题而不是30个问题）。怎么解决？

那么，我怎样才能让两个表都成为现实？如何在查询中减去isUp为真且isUp为假的投票？

Answer 1

您可以选择投票计数：

SELECT Content.* , 
(
  SELECT COUNT(*) 
  FROM VoteQuestion
  WHERE idQuestion = Content.id
) as votes
FROM Content
WHERE type = 'QUESTION'
ORDER BY contentDate
LIMIT 30

Answer 2

您需要一个外部联接：

WITH question AS (
  SELECT * 
  FROM "Content" 
  WHERE "type" = 'QUESTION'
  ORDER BY "contentDate"
  LIMIT 30
)
SELECT COUNT("VoteQuestion"."idUser") 
FROM question
  LEFT JOIN "VoteQuestion" ON "idQuestion" = question."id"
GROUP BY question."id";

在多行中显示值0的COUNT（*）列

2 个答案: