在SQL语句中定义一个CASE（需要简化）

SQL语句（SQL Server）可以工作，但它非常冗余，而且效率似乎很低。我想知道是否有一种方法可以在SELECT语句中定义case语句，然后创建条件语句

我从一个表中提取信息，抓取主键INT ID并检查该ID是否存在于另一个表中。如果没有，则指定一个0值-但如果它确实获取ID，则重复相同的过程以检查另一个表

有人能解释一下如何简化这句话吗

SELECT 
    a.[ID] AS ID, 
    a.[JOB_NUMBER] AS JOB_NUMBER, 

    -- Test to see if we have any entries in job_links_data, if not then assign 0
    CASE WHEN
       (SELECT COUNT([ID]) FROM job_links_data WHERE ([JOB_ID]=a.[ID])) = 0
    THEN 0
    ELSE
       (SELECT COUNT([ID]) FROM job_links_data WHERE ([JOB_ID]=a.[ID]))  
    END AS JOB_LINKS,

    -- If above is 0, then assign an empty string '' - else grab the title
    CASE WHEN
       (SELECT COUNT([ID]) FROM job_links_data WHERE ([JOB_ID]=a.[ID])) = 0
    THEN  ''
    ELSE
       (SELECT c.[TITLE] FROM 
            job_links_info c, 
            job_links_data b, 
            tbl_jobs a 
        WHERE (b.[JOB_LINK_INFO_ID]=c.[ID] AND b.[JOB_ID]=a.[ID]))
    END AS TITLE

FROM tbl_jobs a
WHERE (
    a.[PROJECT_ID]=25  
);

这似乎很愚蠢：

CASE WHEN
   (SELECT COUNT([ID]) FROM job_links_data WHERE ([JOB_ID]=a.[ID])) = 0
THEN 0
ELSE
   (SELECT COUNT([ID]) FROM job_links_data WHERE ([JOB_ID]=a.[ID]))  
END AS JOB_LINKS,

如果值为0，则指定0？替换为：

(SELECT COUNT([ID]) FROM job_links_data WHERE ([JOB_ID]=a.[ID]))  AS JOB_LINKS

我还建议您使用正确、明确、标准的

JOIN

语法。切勿在FROM子句中使用逗号。

这似乎很愚蠢：

CASE WHEN
   (SELECT COUNT([ID]) FROM job_links_data WHERE ([JOB_ID]=a.[ID])) = 0
THEN 0
ELSE
   (SELECT COUNT([ID]) FROM job_links_data WHERE ([JOB_ID]=a.[ID]))  
END AS JOB_LINKS,

如果值为0，则指定0？替换为：

(SELECT COUNT([ID]) FROM job_links_data WHERE ([JOB_ID]=a.[ID]))  AS JOB_LINKS

---

我还建议您使用正确、明确、标准的

JOIN

语法。不要在<代码>中使用逗号从子句。

< P>你应该考虑声明变量并在查询中使用它，类似这样的事情：

DECLARE @JOB_LINKS int
SET @JOB_LINKS = (SELECT COUNT([ID]) FROM job_links_data WHERE ([JOB_ID]=a.[ID]))

-- And after this, your query with the variable.

SELECT 
    a.[ID] AS ID, 
    a.[JOB_NUMBER] AS JOB_NUMBER, 
    @JOB_LINKS AS JOB_LINKS,

    CASE WHEN @JOB_LINKS = 0
    THEN  ''
    ELSE -- selecting the title only when job links is not 0
       (SELECT c.[TITLE] FROM 
            job_links_info c, 
            job_links_data b, 
            tbl_jobs a 
        WHERE (b.[JOB_LINK_INFO_ID]=c.[ID] AND b.[JOB_ID]=a.[ID]))
    END AS TITLE

FROM tbl_jobs a
WHERE (
    a.[PROJECT_ID]=25  
);

---

在语句之前设置变量<强>必须/<强>，这并不总是取决于您如何执行查询。

< P>您应该考虑声明变量并在查询中使用它，例如：

DECLARE @JOB_LINKS int
SET @JOB_LINKS = (SELECT COUNT([ID]) FROM job_links_data WHERE ([JOB_ID]=a.[ID]))

-- And after this, your query with the variable.

SELECT 
    a.[ID] AS ID, 
    a.[JOB_NUMBER] AS JOB_NUMBER, 
    @JOB_LINKS AS JOB_LINKS,

    CASE WHEN @JOB_LINKS = 0
    THEN  ''
    ELSE -- selecting the title only when job links is not 0
       (SELECT c.[TITLE] FROM 
            job_links_info c, 
            job_links_data b, 
            tbl_jobs a 
        WHERE (b.[JOB_LINK_INFO_ID]=c.[ID] AND b.[JOB_ID]=a.[ID]))
    END AS TITLE

FROM tbl_jobs a
WHERE (
    a.[PROJECT_ID]=25  
);

---

变量必须在语句之前设置，这并不总是可能的，这取决于您执行查询的方式。

同意。这很愚蠢，我确实纠正了你的错误。但有没有办法将“JOB_LINKS”分配给一个变量，这样我就不必在下一个案例陈述中再次检查它了？同意。这很愚蠢，我确实纠正了你的错误。但是有没有一种方法可以将“JOB_LINKS”分配给一个变量，这样我就不必在下一个CASE语句中再次检查它了？