Question

SQL语句（SQL Server）可以工作-但它非常冗余并且效率低下。我想知道是否有一种方法可以在SELECT中定义一个case语句，然后创建条件语句。

我正在从一个表中提取信息，获取主键INT ID并检查该ID是否存在于另一个表中。如果不是，则分配一个0值-但如果确实捕获了ID。然后，我重复相同的过程以检查另一个表。

有人可以阐明如何简化此声明吗？

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  
);

Answer 1

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，则分配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的，明确的，标准 JOIN语法。从不在FROM子句中使用逗号。

Answer 2

您应该考虑声明一个变量并在查询中使用它，如下所示：

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  
);

必须在语句前设置变量，但这并非总是可能的，具体取决于您执行查询的方式。

在SQL语句中定义CASE（需要简化）

2 个答案: