一、问题

用以下sql语句查询数据，结果为空

SELECT  a.ID ,
        a.Sub_Project_Name ,
        a.Sub_Project_Type
FROM    TB_KYSubProject a
WHERE   a.ID NOT IN (
        SELECT DISTINCT c.SubprojectID
        FROM    TB_KYGrogramme c
        WHERE   ISNULL(c.Belong_Programme, ‘‘) <> ‘‘
                AND c.Belong_Programme IN ( SELECT  ConfigValue
                                            FROM    PB_Config
                                            WHERE   ConfigKey = ‘子项目共有所属方案‘ ) )

但是查询TB_KYGrogramme和TB_KYSubProject都有数据，TB_KYSubProject比TB_KYGrogramme的数据还多，不应该没数据

TB_KYGrogramme

SELECT DISTINCT c.SubprojectID
        FROM    TB_KYGrogramme c
        WHERE   ISNULL(c.Belong_Programme, ‘‘) <> ‘‘
                AND c.Belong_Programme IN ( SELECT  ConfigValue
                                            FROM    PB_Config
                                            WHERE   ConfigKey = ‘子项目共有所属方案‘ )

TB_KYSubProject

SELECT DISTINCT a.ID
FROM    TB_KYSubProject a

后面筛选TB_KYGrogramme，有一条数据SubprojectID是NULL

SELECT DISTINCT c.SubprojectID
        FROM    TB_KYGrogramme c
        WHERE   ISNULL(c.Belong_Programme, ‘‘) <> ‘‘
                AND c.Belong_Programme IN ( SELECT  c.ConfigValue
                                            FROM    PB_Config c
                                            WHERE   c.ConfigKey = ‘子项目共有所属方案‘ )
                                            AND c.SubprojectID  IS NULL

二、原因

SELECT  1 AS id ,
        1 AS subprojectid
UNION
SELECT  2 AS id ,
        2 AS subprojectid
UNION
SELECT  3 AS id ,
        3 AS subprojectid
UNION
SELECT  4 AS id ,
        NULL AS subprojectid

SELECT  *
FROM    ( SELECT    1 AS id ,
                    1 AS subprojectid
          UNION
          SELECT    2 AS id ,
                    2 AS subprojectid
          UNION
          SELECT    3 AS id ,
                    3 AS subprojectid
          UNION
          SELECT    4 AS id ,
                    NULL AS subprojectid
        ) vv
WHERE   subprojectid NOT IN ( 1, NULL )

等同

SELECT  *
FROM    ( SELECT    1 AS id ,
                    1 AS subprojectid
          UNION
          SELECT    2 AS id ,
                    2 AS subprojectid
          UNION
          SELECT    3 AS id ,
                    3 AS subprojectid
          UNION
          SELECT    4 AS id ,
                    NULL AS subprojectid
        ) vv
WHERE   subprojectid <> 1
        AND subprojectid <> NULL 

NULL值不能参与比较运算符，要筛选非NULL数据，要用 is not null，而不能用<>NULL，具体看下数据库中的三值逻辑（Tree-Value-Logic）。

所以子查询有结果是NULL，那查询条件为空

三、方法

知道问题原因，对sql语句修改，把SubprojectID是NULL的数据排查

SELECT  a.ID ,
        a.Sub_Project_Name ,
        a.Sub_Project_Type
FROM    TB_KYSubProject a
WHERE   a.ID NOT IN (
        SELECT DISTINCT c.SubprojectID
        FROM    TB_KYGrogramme c
        WHERE   ISNULL(c.Belong_Programme, ‘‘) <> ‘‘
                AND c.Belong_Programme IN ( SELECT  ConfigValue
                                            FROM    PB_Config
                                            WHERE   ConfigKey = ‘子项目共有所属方案‘ )  AND c.SubprojectID IS NOT NULL)

开发中遇到该问题记录下

原文地址：https://www.cnblogs.com/Zev_Fung/p/8488107.html