Question

以下查询不会失败，但也不会插入唯一值。

我只想插入唯一值

DECLARE @user_id AS varchar(80) 
DECLARE @real_name as varchar(250)

- 新的唯一值是“aaa”，它们在目标表中不存在

SET @user_id = 'aaa'
SET @real_name = 'aaa'

INSERT INTO TargetTable
  ([user_id],real_name)
SELECT @user_id, @real_name
WHERE NOT EXISTS
  (SELECT [user_id],real_name FROM TargetTable)

Answer 1

如果您按照自己的方式遇到插入问题，请尝试不同的方法：

-- if the count of records with that @user_id and @real_name is less or equal to 0, 
-- that means it's not in the table
IF (0 >= (SELECT COUNT(*) FROM TargetTable WHERE @user_id = [user_id] AND @real_name = [real_name]))
BEGIN
   -- then call a normal insert
   INSERT
     INTO TargetTable ([user_id], real_name )
   VALUES             (@user_id , @real_name)

END

Answer 2

查询插入行的唯一方法是，如果数据库上没有'aaa'，'aaa'的行。

看，你在做什么

SELECT 'aaa','aaa'
WHERE NOT EXISTS
(SELECT [user_id],real_name FROM TargetTable)

所以，它的'aaa','aaa'结果集“减去”或“除了”由目标表上的每一行的user_id和real_name组成的结果集。

因此，如果'aaa'，'aaa'已存在于第二个结果集中，它将从最终结果集中排除，因此不会被插入

Answer 3

如果有人还在乎，或者稍后再看这个问题......

最初的问题是OP使用WHERE NOT EXISTS子句来检查目标表中存在的任何行，而不是特定的行。只需检查您要插入的数据：

DECLARE @user_id AS varchar(80)  
DECLARE @real_name as varchar(250)  

 --New Unique values are "aaa" and they do not exist in the target table
SET @user_id = 'aaa' 
SET @real_name = 'aaa' 

INSERT INTO TargetTable 
  ([user_id],real_name) 
SELECT @user_id, @real_name 
WHERE NOT EXISTS 
  ( SELECT 42 FROM TargetTable WHERE [user_id] = @user_id AND real_name = @real_name )

（当前）接受的答案鼓励不安全的设计，其中执行两个单独的查询而没有TRANSACTION的好处。查询之间发生的更改可能会导致令人不快的结果。

Answer 4

这是为了解决user92546的问题。插入发生在查询TargetTable的同一查询中。唯一的子选择是针对变量，它们显然不会在此插入过程中发生变化，而表格可能会在任何微秒级发生变化。

DECLARE @user_id AS varchar(80)  
DECLARE @real_name as varchar(250)  

 --New Unique values are "aaa" and they do not exist in the target table
SET @user_id = 'aaa' 
SET @real_name = 'aaa' 

INSERT INTO TargetTable ([user_id],real_name) 
SELECT T1.user_id, T1.real_name 
FROM (select @user_id [user_id], @real_name [real_name] ) T1
LEFT JOIN TargetTable T2
   on T1.user_id = T2.user_id
     and t1.real_name = T2.real_name
     and T2.user_id = @user_id
     and T2.real_name = @real_name
WHERE T2.user_id is null

使用不存在的参数插入

4 个答案: