在 SQL 中将 AND 或 OR 放在一个条件中

Posted 2023-05-08

【中文标题】在 SQL 中将 AND 或 OR 放在一个条件中

【英文标题】：Putting AND or OR in one condition in SQL

【发布时间】：2020-09-29 13:37:29

【问题描述】：

我有一个EMPLOYEE 表，它返回一个EMPLOYEE_type。 EMPLOYEE_type 的值可以是 1 或 2。

我有另一个表DISCOUNT，其中我们有列DISCOUNT_EMPLOYEE_TYPE。 DISCOUNT_EMPLOYEE_TYPE 的值可以是 1、2 或 NULL（不存在）。

我必须找到下面的结果。

如果EMPLOYEE_type = 2 则显示所有DISCOUNT 和DISCOUNT_EMPLOYEE_TYPE = 2。

如果EMPLOYEE_type = 1 则显示所有DISCOUNT 与DISCOUNT_EMPLOYEE_TYPE = 1 或NULL。

到目前为止，我已经尝试过以下查询：

DECLARE @employee_type smallint 

SET @employee_type = (SELECT employee_type FROM [dbo].[EMPLOYEE] (nolock)
                AND employee_stub = @employee_stub)

SELECT *
FROM [dbo].[DISCOUNT] (nolock)
WHERE acct_id = @acct_id 
  AND (discount_employee_type = @employee_type
       OR (@employee_type IS NULL AND @employee_type = 1))

如果@employee_type = 2返回正确的结果。

但是，在 @employee_type = 1 的情况下它不会返回 NULL 值 - 我只返回带有 discount_employee_type = 1 的行。

【问题讨论】：

= null?那永远不会返回true。你想要is null。

我也试过 wilh is null，它还没有给出结果

其实(@employee_type is null and @employee_type=1)永远不会成功

你为什么对你的两个表都使用NOLOCK？您对可能存在错误的数据感到满意吗？

【参考方案1】：

根据您的描述，我建议 WHERE 子句应该是

WHERE
acct_id = @acct_id 
and (
    (employee_type = 2 AND discount_employee_type = 2)
    OR (@employee_type = 1 AND (discount_employee_type = 1 OR discount_employee_type IS NULL))
    )

【参考方案2】：

在 WHERE 子句中使用 CASE 表达式来检查 2 个条件中的每一个：

SELECT *
FROM [dbo].[DISCOUNT] 
WHERE acct_id = @acct_id
AND 1 = CASE
  WHEN @employee_type = 1 AND COALESCE(discount_employee_type, 1) = 1 THEN 1
  WHEN @employee_type = 2 AND discount_employee_type = 2 THEN 1 
END 

【参考方案3】：

更正了我的代码：

declare @employee_type int
declare @discount table
(
col1 int
,discount_employee_type varchar(1)
)

--set @employee_type = 1
set @employee_type = 2

insert into @discount(col1,discount_employee_type)
values (1,'1'), (2,'1'), (3,'2'), (4,null), (5,'2')

select * from @discount

select * 
from
@discount

WHERE

1 = 

CASE 
    WHEN @employee_type = 1 and (discount_employee_type is null or discount_employee_type = 1) then 1
    WHEN @employee_type =  2 and  discount_employee_type = 2 THEN 1
    ELSE 0
END

DBFiddle link here