mysql - 如何在 mySQL 中选择异常

Question

从 myTable 中选择部门，其中联系人为空

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

在上表中，如何选择联系人为空的部门，不包括现有联系人。

上面的语句将返回“清洁”和“硬件”。

我所期望的只是“硬件”

我怎样才能只选择“硬件”或者有可能。

TIA

Answer 1

我认为您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，如果您想在没有该IN子句的情况下执行此操作：

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

NOT NULL使用for 条件进行自连接，然后JOIN通过检查子句NULL来验证失败。WHERE

Answer 2

听起来您想排除与 ANY 记录中的非空值关联的部门。为此，您应该执行子查询以获取所有非空部门，并将它们从检索空部门的查询中排除。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 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 ;