sql - 计算两个表的行数

Question

我想计算两个表的行数，我写了这段代码：

SELECT count(v.id), count(c.id)
FROM votes as v, content as c
WHERE v.user_id=1 AND c.created_by=1

它正确返回一行和两列，只有这两个单元格具有完全相同的值......而且不应该如此

Answer 1

您的查询正在两个表之间进行交叉连接。如果要计算行数，则需要以下内容：

select 'vote' as which, count(*)
from votes v
where v.user_id = 1
union all
select 'content' as which, count(*)
from content c
where c.created_by = 1

如果您正在寻找一行、两列，请改用交叉连接：

select vcnt, ccnt
from (select count(*) as ccnt
      from votes v
      where v.user_id = 1
     ) v cross join
     (select count(*) as ccnt
      from content c
      where c.created_by = 1
     ) c

Answer 2

您需要计算不同的值，否则您只需计算相应列中包含 ID 的行数（这将是所有行，除非您NULL在某些 ID 中有 s）：

count(DISTINCT v.id), count(DISTINCT c.id)

Answer 3

仅当所选 id 不为 null 时才计数

SELECT SUM(ISNULL(v.id,0,1)), SUM(ISNULL(c.id,0,1))
FROM votes as v, content as c
WHERE v.user_id=1 AND c.created_by=1

但是如果你需要单独计算，按照这个。

SELECT 
  (SELECT COUNT(*) FROM votes as v WHERE v.user_id=1) AS Count1,
  (SELECT COUNT(*) FROM content as c WHERE c.created_by=1) AS Count2

Answer 4

SELECT
    COUNT(Votes.id) VotesCount,
    COUNT(Content.id) ContentCount
FROM
    Votes 
    FULL OUTER JOIN Content ON 1=2 --Ensures rows never join

这会将两个表连接在一起，但不匹配任何行。因此，对于 Votes 中的每一行，所有 Content 列都将为 NULL，反之亦然。COUNT(ColumnName) 不计算 NULL 值，但 COUNT(*) 计算。因此，这应该给你你的结果。

Answer 5

如果您希望数据在同一行中，则可以使用 aUNION ALL和 thenCASE语句：

select max(case when col = 'voteCount' then cnt end) as voteCount,
     max(case when col = 'ContentCount' then cnt end) as ContentCount
from
(
    select count(*) cnt, 'voteCount' col
    from votes v
    where v.user_id = 1
    union all
    select count(*) cnt, 'ContentCount' col
    from content c
    where c.created_by = 1
) x