问题
I wrote this SQL in a stored procedure but not working,
declare @tableName varchar(max) = 'TblTest'
declare @col1Name varchar(max) = 'VALUE1'
declare @col2Name varchar(max) = 'VALUE2'
declare @value1 varchar(max)
declare @value2 varchar(200)
execute('Select TOP 1 @value1='+@col1Name+', @value2='+@col2Name+' From '+ @tableName +' Where ID = 61')
select @value1
execute('Select TOP 1 @value1=VALUE1, @value2=VALUE2 From TblTest Where ID = 61')
This SQL throws this error:
Must declare the scalar variable "@value1".
I am generating the SQL dynamically and I want to get value in a variable. What should I do?
回答1:
The reason you are getting the DECLARE error from your dynamic statement is because dynamic statements are handled in separate batches, which boils down to a matter of scope. While there may be a more formal definition of the scopes available in SQL Server, I've found it sufficient to generally keep the following three in mind, ordered from highest availability to lowest availability:
Global:
Objects that are available server-wide, such as temporary tables created with a double hash/pound sign ( ##GLOBALTABLE, however you like to call # ). Be very wary of global objects, just as you would with any application, SQL Server or otherwise; these types of things are generally best avoided altogether. What I'm essentially saying is to keep this scope in mind specifically as a reminder to stay out of it.
IF ( OBJECT_ID( 'tempdb.dbo.##GlobalTable' ) IS NULL )
BEGIN
CREATE TABLE ##GlobalTable
(
Val BIT
);
INSERT INTO ##GlobalTable ( Val )
VALUES ( 1 );
END;
GO
-- This table may now be accessed by any connection in any database,
-- assuming the caller has sufficient privileges to do so, of course.
Session:
Objects which are reference locked to a specific spid. Off the top of my head, the only type of session object I can think of is a normal temporary table, defined like #Table. Being in session scope essentially means that after the batch ( terminated by GO ) completes, references to this object will continue to resolve successfully. These are technically accessible by other sessions, but it would be somewhat of a feat do to so programmatically as they get sort of randomized names in tempdb and accessing them is a bit of a pain in the ass anyway.
-- Start of session;
-- Start of batch;
IF ( OBJECT_ID( 'tempdb.dbo.#t_Test' ) IS NULL )
BEGIN
CREATE TABLE #t_Test
(
Val BIT
);
INSERT INTO #t_Test ( Val )
VALUES ( 1 );
END;
GO
-- End of batch;
-- Start of batch;
SELECT *
FROM #t_Test;
GO
-- End of batch;
Opening a new session ( a connection with a separate spid ), the second batch above would fail, as that session would be unable to resolve the #t_Test object name.
Batch:
Normal variables, such as your @value1 and @value2, are scoped only for the batch in which they are declared. Unlike #Temp tables, as soon as your query block hits a GO, those variables stop being available to the session. This is the scope level which is generating your error.
-- Start of session;
-- Start of batch;
DECLARE @test BIT = 1;
PRINT @test;
GO
-- End of batch;
-- Start of batch;
PRINT @Test; -- Msg 137, Level 15, State 2, Line 2
-- Must declare the scalar variable "@Test".
GO
-- End of batch;
Okay, so what?
What is happening here with your dynamic statement is that the EXECUTE() command effectively evaluates as a separate batch, without breaking the batch you executed it from. EXECUTE() is good and all, but since the introduction of sp_executesql(), I use the former only in the most simple of instances ( explicitly, when there is very little "dynamic" element of my statements at all, primarily to "trick" otherwise unaccommodating DDL CREATE statements to run in the middle of other batches ). @AaronBertrand's answer above is similar and will be similar in performance to the following, leveraging the function of the optimizer when evaluating dynamic statements, but I thought it might be worthwhile to expand on the @param, well, parameter.
IF NOT EXISTS ( SELECT 1
FROM sys.objects
WHERE name = 'TblTest'
AND type = 'U' )
BEGIN
--DROP TABLE dbo.TblTest;
CREATE TABLE dbo.TblTest
(
ID INTEGER,
VALUE1 VARCHAR( 1 ),
VALUE2 VARCHAR( 1 )
);
INSERT INTO dbo.TblTest ( ID, VALUE1, VALUE2 )
VALUES ( 61, 'A', 'B' );
END;
SET NOCOUNT ON;
DECLARE @SQL NVARCHAR( MAX ),
@PRM NVARCHAR( MAX ),
@value1 VARCHAR( MAX ),
@value2 VARCHAR( 200 ),
@Table VARCHAR( 32 ),
@ID INTEGER;
SET @Table = 'TblTest';
SET @ID = 61;
SET @PRM = '
@_ID INTEGER,
@_value1 VARCHAR( MAX ) OUT,
@_value2 VARCHAR( 200 ) OUT';
SET @SQL = '
SELECT @_value1 = VALUE1,
@_value2 = VALUE2
FROM dbo.[' + REPLACE( @Table, '''', '' ) + ']
WHERE ID = @_ID;';
EXECUTE dbo.sp_executesql @statement = @SQL, @param = @PRM,
@_ID = @ID, @_value1 = @value1 OUT, @_value2 = @value2 OUT;
PRINT @value1 + ' ' + @value2;
SET NOCOUNT OFF;
回答2:
Declare @v1 varchar(max), @v2 varchar(200);
Declare @sql nvarchar(max);
Set @sql = N'SELECT @v1 = value1, @v2 = value2
FROM dbo.TblTest -- always use schema
WHERE ID = 61;';
EXEC sp_executesql @sql,
N'@v1 varchar(max) output, @v2 varchar(200) output',
@v1 output, @v2 output;
You should also pass your input, like wherever 61 comes from, as proper parameters (but you won't be able to pass table and column names that way).
回答3:
Here is a simple example :
Create or alter PROCEDURE getPersonCountByLastName (
@lastName varchar(20),
@count int OUTPUT
)
As
Begin
select @count = count(personSid) from Person where lastName like @lastName
End;
Execute below statements in one batch (by selecting all)
1. Declare @count int
2. Exec getPersonCountByLastName kumar, @count output
3. Select @count
When i tried to execute statements 1,2,3 individually, I had the same error. But when executed them all at one time, it worked fine.
The reason is that SQL executes declare, exec statements in different sessions.
Open to further corrections.
回答4:
This will occur in SQL Server as well if you don't run all of the statements at once. If you are highlighting a set of statements and executing the following:
DECLARE @LoopVar INT
SET @LoopVar = (SELECT COUNT(*) FROM SomeTable)
And then try to highlight another set of statements such as:
PRINT 'LoopVar is: ' + CONVERT(NVARCHAR(255), @LoopVar)
You will receive this error.
来源：https://stackoverflow.com/questions/21363415/must-declare-the-scalar-variable