如何在 WHERE 子句中使用 CASE/IF 语句？

Question

我声明了 2 个变量，即 @empCode 和 @centerCode。一个中心包含多名员工。我有另一个变量是@IsEmployeeBased 现在如果@IsEmployeeBased = 1，那么它将使用@empCode，否则它将使用@centerCode。 我的查询是这样的。

    DECLARE @IsEmployeeBased INT = 1;
    DECLARE @empCode INT = 1;
    DECLARE @centerCode INT = 3;
    SELECT * FROM Employee
    WHERE Department = 'Information Tech'
    AND 
    CASE WHEN @IsEmployeeBased = 1 THEN ID = @empCode ELSE CenterCode = @centerCode

这是返回错误，我认为 CASE 在 where 子句中是不允许的。有什么方法可以让所需的查询正常工作吗？

这里是示例数据：

CREATE TABLE Employee
(
ID Int,
EmployeeName VARCHAR(25),
Department VARCHAR(25),
CenterCode INT 
)

INSERT INTO Employee VALUES (1, 'Asim', 'Information Tech', 4)
INSERT INTO Employee VALUES (2, 'Ali', 'Information Tech', 2)
INSERT INTO Employee VALUES (3, 'Isaac', 'Information Tech', 3)
INSERT INTO Employee VALUES (4, 'Swagger', 'Information Tech', 4)
INSERT INTO Employee VALUES (5, 'Nadine', 'Information Tech', 2)
INSERT INTO Employee VALUES (6, 'Julie', 'Information Tech', 4)
INSERT INTO Employee VALUES (7, 'Meachum', 'Information Tech', 3)
INSERT INTO Employee VALUES (8, 'Bob Lee', 'Information Tech', 4)

Answer 1

CASE 表达式的谓词（即THEN 或ELSE 之后的内容）必须是一个常量值，而不是另一个逻辑表达式。您可以改写 WHERE 子句以不使用 CASE 表达式：

WHERE
    Department = 'Information Tech' AND (
    (@IsEmployeeBased = 1 AND ID = @empCode) OR
    (@IsEmployeeBased <> 1 AND enterCode = @centerCode))

Answer 2

您也可以尝试创建动态可执行查询，如下所示。

DECLARE @IsEmployeeBased INT = 0;
DECLARE @empCode INT = 1;
DECLARE @centerCode INT = 3;

DECLARE @Query NVarchar(500)
SET @Query = N'SELECT * FROM Employee WHERE Department = ''Information Tech'''
DECLARE @Where Varchar(250) = ''
IF (@IsEmployeeBased = 1)
BEGIN
    SET @Where += 'AND ID = ' + Convert(char(5), @empCode) +''
END
ELSE IF(@IsEmployeeBased <> 1)
BEGIN
    SET @Where += 'AND CenterCode = ' + Convert(char(5), @centerCode) +''
END
SET @Query = @Query + @Where 

--Select @Query
EXECUTE sp_executesql @Query

也请查看此thread。