如何使用 SQL 中的 Case 语句将数据插入临时表

Question

我正在尝试使用Case 语句将数据插入到临时表中，下面是我的代码，因此我收到此错误

子查询返回超过 1 个值。当子查询跟随 =、!=、、>= 或子查询用作表达式时，这是不允许的。

我无法弄清楚我做错了什么。这是根据Case 语句中提供的特定条件将数据插入临时表的正确方法吗？

DECLARE @temp_idCol TABLE 
                    (
                         uid int IDENTITY(1, 1),
                         id nvarchar(20)
                    )
  
DECLARE @_ThresholdOperator nvarchar(20) = '=', 
        @_Threshold_Val nvarchar(20) = '0.4' 

INSERT INTO @temp_idCol (id) 
    SELECT
        CASE
            WHEN @_ThresholdOperator = '=' 
                THEN (SELECT q.id 
                      FROM [dbo].[Summit_Twitter_HashtagDetails] q  
                      WHERE q.Ticket_ID IS NULL
                        AND q.SentimentAnalyis_Score = @_Threshold_Val   
                         OR q.IsProcessedOrNot = 0)
            WHEN @_ThresholdOperator = '>' 
                THEN (SELECT q.id 
                      FROM [dbo].[Summit_Twitter_HashtagDetails] q  
                      WHERE q.Ticket_ID IS NULL
                        AND q.SentimentAnalyis_Score > @_Threshold_Val   
                         OR q.IsProcessedOrNot = 0)
            WHEN @_ThresholdOperator = '<' 
                THEN (SELECT q.id 
                      FROM [dbo].[Summit_Twitter_HashtagDetails] q  
                      WHERE q.Ticket_ID IS NULL
                        AND q.SentimentAnalyis_Score < @_Threshold_Val    
                         OR q.IsProcessedOrNot = 0)  
        END AS id 
    FROM
        [dbo].[Summit_Twitter_HashtagDetails] 

Answer 1

CASE 表达式只能返回 1 个值。

但是你可以使用IF和ELSE

简化示例：

create table test (id int identity primary key, col decimal(9,1));
insert into test (col) values (0.1),(0.2),(0.3),(0.4),(0.5);

declare @ids table (id int);
declare @_ThresholdOperator varchar(2) = '>', 
        @_Threshold_Val nvarchar(20) = '0.3';

IF @_ThresholdOperator = '='
  insert into @ids (id) 
  select id from test where col = @_Threshold_Val;
ELSE IF @_ThresholdOperator = '<'
  insert into @ids (id) 
  select id from test where col < @_Threshold_Val;
ELSE IF @_ThresholdOperator = '>'
  insert into @ids (id) 
  select id from test where col > @_Threshold_Val;
ELSE BEGIN 
     insert into @ids (id) values (0);
     update @ids set id = id + 42;
     END;

select * from @ids;

id
4
5

db小提琴here

额外

CASE 仍然可以在 WHERE 子句中使用。
只要它只返回 1 个值。
它只是不可分割。

  insert into @ids (id) 
  select id from test 
  where case @_ThresholdOperator 
        when '=' then iif(col=@_Threshold_Val,1,0)
        when '<' then iif(col<@_Threshold_Val,1,0)
        when '>' then iif(col>@_Threshold_Val,1,0)
        end = 1;