T-SQL存储过程 - 无法将输出值分配给变量

Question

我有一个存储过程，找到当前的BatchID并将其递增1并返回该值作为输出参数;但是，每当我在它之后使用select语句时，我会在执行存储过程之前将值设置为它。

为什么会这样？

执行存储过程：

declare @BatchStartTS datetime = getdate()
declare @NewBatchID int = 0

exec dbo.[usp_CreateBatchRecord] @BatchStartTS, @NewBatchID output

select [NewBatchID] = @NewBatchID

存储过程：

IF OBJECT_ID('dbo.usp_CreateBatchRecord') IS NOT NULL 
     DROP PROCEDURE dbo.usp_CreateBatchRecord
GO

CREATE PROCEDURE dbo.usp_CreateBatchRecord
    @BatchDate DATETIME = NULL,
    @BatchID INT OUTPUT
AS
BEGIN
    /*  PROCEDURE       dbo.[usp_CreateBatchRecord]
        DESCRIPTION:    Creates a new Import Batch record and returns the next BatchID
        PARAMETERS
         + Required:    N/A
         + Optional:    @BatchDate
        OUTPUT:         @BatchID - returns the BatchID of the new Batch record created

        MODIFIED:       N/A

        EXAMPLE(S):
                    -- must set optional parameter to DEFAULT value AND declare output parameter
                    declare @NewBatchID int = null
                    exec dbo.[usp_CreateBatchRecord] DEFAULT,@NewBatchID OUTPUT

                    -- must declare output parameter
                    declare @NewBatchID int = null
                    exec dbo.[usp_CreateBatchRecord] '2016-06-26 15:11:00',@NewBatchID OUTPUT
    */

    SET NOCOUNT ON;

    DECLARE @ErrMsg VARCHAR(MAX) = NULL
    DECLARE @ErrNum INT = NULL
    DECLARE @ErrLineNo INT = NULL

    /* PARAMETER CHECKING */
    if ISNULL(LEN(@BatchDate), 0) = 0
         SET @BatchDate = GETDATE()

    IF OBJECT_ID('dbo.Batch') IS NULL 
    BEGIN
        SET @ErrMsg = ERROR_MESSAGE()
        SET @ErrLineNo = ERROR_LINE()
        SET @ErrNum = ERROR_NUMBER()

        GOTO ERR_HANDLING
    END

    /* INTERNAL VARIABLES */
    DECLARE @CurrentMaxBatchID INT = (SELECT MAX(BatchID) FROM dbo.Batch)
    DECLARE @NextBatchID INT = @CurrentMaxBatchID + 1

    BEGIN TRY
        /* ACTION */
        INSERT INTO dbo.Batch (BatchID, BatchStartTS)
            SELECT @NextBatchID, @BatchDate

        /* RETURN VALUE(S) */
        SELECT @NextBatchID

        GOTO THE_END
    END TRY
    BEGIN CATCH
        SET @ErrMsg = ERROR_MESSAGE()
        SET @ErrLineNo = ERROR_LINE()
        SET @ErrNum = ERROR_NUMBER()
    END CATCH

ERR_HANDLING:
    RAISERROR('<!> ERROR:  (%i) [%i] %s',16,1,@ErrNum,@ErrLineNo,@ErrMsg) WITH NOWAIT

THE_END:
    RETURN
END