我怎样才能在mysql用例外选择

Question

SELECT部门FROM mytable的WHERE接触IS NULL

Department Contact 
    ------------- --------- 
    Cleaning  Jack 
    Admin   Peter 
    Software  James 
    Cleaning  (null) 
    Cleaning  Jill 
    Hardware  (null) 

在上表中，如何选择部门，在那里接触是空排除存在接触。

以上声明 将返回“清洗”和“硬件”。

我的预期是什么只有“硬件”

我怎么可以只选择“硬件”还是可能的。

TIA

Answer 1

这听起来像你想排除与任何记录一个非空值相关部门。为此，您应该执行子查询来获取所有非空的部门，并从检索空部门的查询中排除它们。像这样：

SELECT DISTINCT department 
FROM mytable 
WHERE contact is null 
AND department NOT IN (SELECT department FROM mytable where contact is not null) 

Answer 2

我想你只想返回没有任何非空联系人的行。

SELECT 
    DISTINCT department 
FROM 
    myTable 
WHERE 
    contact IS NULL 
    AND department NOT IN (SELECT department FROM myTable where contact IS NOT NULL) 

，或者，如果你想这样做没有IN条款：

SELECT 
    DISTINCT a.[department] 
FROM 
    myTable AS a 
LEFT OUTER JOIN 
    myTable AS b 
     ON b.[department] = a.[department] 
     AND b.[contact] IS NOT NULL 
WHERE 
    a.[contact] IS NULL 
    AND b.[contact] IS NULL 

自联接使用NOT NULL为条件，然后验证JOIN未能在WHERE子句中的NULL检查。

Answer 3

SELECT department, 
     SUM(CASE WHEN Contact is not null THEN 1 ELSE 0 END) as count 
     from myTable 
     group by department 
     HAVING count > 0 ;