为什么这个查询在存储过程中返回不同的结果? [英] Why does this query return different results when in a stored procedure?

问题描述

我有一个带有 WHERE 子句的查询，该子句包含一个条件，该条件根据可能为 NULL 的参数检查 NULLable 列，如下所示:

I've got a query with a WHERE clause that contains a condition that checks a NULLable column against a parameter that may be NULL, like so:

SELECT ...
FROM Table
WHERE NullableColumn = @NullableParameter

根据我的理解，SQL Server 应该总是将 NULL = NULL non-NULL = NULL 评估为 false，这意味着如果 @NullableParameter为 NULL 则上述查询将返回零条记录.

From my understanding, SQL Server is supposed to evaluate NULL = NULL non-NULL = NULL as false always, meaning if @NullableParameter is NULL then the above query would return zero records.

这是我在查询窗口中运行查询时看到的行为，但是当我在存储过程中使用相同的查询并且 @NullableParameter 为 NULL 时，它会返回 所在的所有记录NullableColumn 为 NULL.简而言之，sproc 似乎在说 NULL = NULL 是真的.

That's the behavior I see when running the query in a query window, but when I use the same query in a stored procedure and when @NullableParameter is NULL it is returning all records where NullableColumn is NULL. In short, it seems like the sproc is saying NULL = NULL is true.

这就是问题/问题简而言之.实际的 WHERE 子句涉及更多，在下面的部分中提供了更多详细信息.

That's the problem/question is a nutshell. The actual WHERE clause is a bit more involved and is presented with more detail in the section below.

我有一个数据库，其中包含一个名为 StudyResults 的表.每条记录都由 StudyResultId 主键字段唯一标识.还有一个 ParticipantId 字段，指示参与研究的受试者，以及 GroupId，标识受试者所属的组(如果有).如果研究是单人研究，GroupId 为 NULL.ParticipantId 不能为 NULL.

I have a database with a table called StudyResults. Each record is uniquely identified by a StudyResultId primary key field. There is also a ParticipantId field, that indicates the subject involved in the study, and GroupId, which identifies what group the subject belonged to, if any. If the study was a single-person study, GroupId is NULL. ParticipantId cannot be NULL.

我有一个存储过程，需要为特定研究更新 StudyResults 表中的一些记录，但问题是——如果研究是单人研究，那么我只需要更新它一排；如果是小组研究，我想为该小组更新StudyResults 中针对该主题的所有行.

I have a stored procedure that needs to update some records in the StudyResults table for a particular study, but here's the rub - if the study is single-person study then I need to just update that one row; if it was a group study I want to update all rows in StudyResults for that group, for that subject.

这并不难实现.存储过程被传递一个 StudyResultId，然后运行以下 T-SQL 来确定该行的 GroupId 和 ParticipantId 值:

This isn't too hard to accomplish. The stored procedure is passed a StudyResultId and then runs the following T-SQL to determine the GroupId and ParticipantId values for that row:

DECLARE @GroupId INT, @ParticipantId INT
SELECT @GroupId = GroupId,
       @ParticipantId = ParticipantId
FROM StudyResults
WHERE StudyResult = @StudyResultId

接下来，我创建一个 CURSOR 来枚举感兴趣的 StudyResults 记录.注意 WHERE 子句，它说，给我记录，其中 StudyResultId 等于传递到 sproc 的 @StudyResultId 或 GroupId 和 ParticipantId 值与 StudyResults 的 GroupId 和 ParticipantId 值对齐 兴趣记录.

Next, I create a CURSOR to enumerate the StudyResults records of interest. Note the WHERE clause, which says, "Give me the records where StudyResultId equals the @StudyResultId passed into the sproc or the GroupId and ParticipantId values line up with the GroupId and ParticipantId values for the StudyResults record of interest.

DECLARE resultsToEnumerate CURSOR LOCAL FAST_FORWARD FOR 
SELECT DISTINCT StudyResultId
FROM StudyResults
WHERE StudyResult = @StudyResultId OR (GroupId = @GroupId AND ParticipantId= @ParticipantId)

如果 @GroupId 为 NULL，那么比较 GroupId = @GroupId 应该总是为假，对吧?因为对于 SQL Server，NULL = NULL 为假，non-NULL = NULL 为假.

If @GroupId is NULL, then comparing GroupId = @GroupId should always be false, right? Because for SQL Server, NULL = NULL is false, and non-NULL = NULL is false.

但这就是奇怪的地方 - 如果我从查询窗口运行上述语句并使用 @StudyResultId 进行单人研究，则 CURSOR 包含我期望的内容 - 即单个记录.但是，如果我将完全相同的代码放在存储过程中并运行它，则 CURSOR 包含该参与者的所有单人研究！就好像它在说 @GroupId 为 NULL，所以我将返回 GroupId 为 NULL 的所有记录，但为什么呢?NULL = NULL 不应该返回记录，对吗?

But here's what it gets weird - if I run the above statements from a query window and use a @StudyResultId for a single-person study the CURSOR contains what I expect - namely, a single record. However, if I put the exact same code in a stored procedure and run it the CURSOR contains all of the single-person studies for that participant! It's as if it is saying @GroupId is NULL, so I'll return all records where GroupId is NULL, but why? NULL = NULL should never return a record, right?

事实上，如果我进入 sproc 并更改 WHERE 子句并将 GroupID = @GroupID 替换为 NULL = NULL 我看到相同的结果 - 所有针对相关参与者的单人研究.所以它显然是在 sproc 中将 NULL = NULL 评估为 true(或忽略它).

In fact, if I go into the sproc and change the WHERE clause and replace GroupID = @GroupID with NULL = NULL I see the same results - all single-person studies for the participant in question. So it clearly is evaluating NULL = NULL to true in the sproc (or ignoring it).

我可以修复"在存储过程中通过在括号中添加额外的检查来确保 GroupId IS NOT NULL 像这样:

I can "fix" this in the stored procedure by adding an additional check in the parentheses to ensure GroupId IS NOT NULL like so:

WHERE ActivityID = @ActivityID OR (GroupID IS NOT NULL AND GroupID = @GroupID AND PatientID = @PatientID)

这就是我所做的，但我很困惑为什么 WHERE 子句在我的存储过程中的计算方式不同.

This is what I have done, but I am perplexed as to why the WHERE clause is being evaluated differently in my stored procedure.

推荐答案

在将 ANSI_NULLS 设置为 ON 后尝试更改会话中的存储过程.

Try altering the stored procedure in a session after setting ANSI_NULLS to ON.

SET ANSI_NULLS ON
GO

alter procedure procedureName 
...

来自文档:

对于存储过程，SQL Server 使用SET ANSI_NULLS 设置值来自的初始创建时间存储过程.每当存储程序随后被执行，SET ANSI_NULLS 的设置是恢复到原来的使用价值并生效.在内部调用时一个存储过程，SET的设置ANSI_NULLS 未更改.

For stored procedures, SQL Server uses the SET ANSI_NULLS setting value from the initial creation time of the stored procedure. Whenever the stored procedure is subsequently executed, the setting of SET ANSI_NULLS is restored to its originally used value and takes effect. When invoked inside a stored procedure, the setting of SET ANSI_NULLS is not changed.

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