Question

我有两个表：Jobs和JobItems：

CREATE TABLE Jobs (
    [JobId] UNIQUEIDENTIFIER NOT NULL PRIMARY KEY
);

Create TABLE JobItems(
    [ItemId] UNIQUEIDENTIFIER NOT NULL,
    [JobId] UNIQUEIDENTIFIER NOT NULL
);

并且对于Jobs中的每一行，JobItems中可以有一行，一行或多行，以使其JobId与Jobs中的行匹配，这意味着零，一个或多个“作业”包含该“项目”。

我需要从Jobs中选择行，以便JobItems中只有一行JobId与Jobs中的行匹配，并尝试来自{{3}的代码（稍微改动一下）：

SELECT JobId, (SELECT Count(ItemId) from JobItems WHERE
     JobItems.JobId=Jobs.JobId) from Jobs

问题是我现在需要以某种方式说我只需要一个匹配的行。所以我尝试为Count(ItemId)

指定一个快捷方式

SELECT JobId, (SELECT Count(ItemId) AS CountItemId from JobItems WHERE
     JobItems.JobId=Jobs.JobId) from Jobs WHERE CountItemId=1

但这会让SQL服务器不满意 - 它说

列名CountItemId无效

如何只选择一个匹配的项目？

Answer 1

基本的GROUP BY HAVING子句会为你做..

      SELECT Jobs.jobId
      FROM Jobs 
      INNER JOIN JobItems
         ON Jobs.JobId = JobItems.JobId
      GROUP BY Jobs.jobId
      HAVING COUNT(*)=1

Answer 2

使用此查询：

SELECT JobId, Count(ItemId) from JobItems group by JobId;

Answer 3

你可以这样做：

select *
from jobs i
where (select count(*) from jobitems ji where ji.jobid = j.jobid) = 1;

但是，如果表作业只包含所有作业ID而不包含任何其他内容，那么您根本不必从表作业中选择：

select jobid
from jobitems 
group by jobid
having count(*) = 1;

Answer 4

Select jobid
From jobitems 
Group by jobid
Having count(*) = 1;

Answer 5

您还可以创建临时结果集：

WITH T1 AS (
  SELECT JobId
  FROM Jobs 
),
T2 AS (
  SELECT JobId, Count(ItemId) AS CountItemId 
  FROM JobItems 
  GROUP BY JobID
)

SELECT JobId, CountItemId
FROM T2
WHERE CountItemId = 1

如何从一个表中选择项目，使另一个表中的查询恰好产生一行？

5 个答案: