带条件插入的存储过程

Question

我正在尝试使用If语句编写过程。

这是我的数据库

我的想法是：当我们拥有相同的ComandmentNumber时，StudentNumber必须增加1，但是当我们拥有新的ComandmentNumber时，StudentNumber必须从1开始。

示例：

这是我在SQL Server中的过程

CREATE PROCEDURE InsertIntoDefeatsGraduates
    @ComandmentNumber VARCHAR(100),
    @StudentFakNumber INT
AS
BEGIN
    DECLARE @StudentNumber INT 

    IF EXISTS (SELECT * FROM dbo.[Defeats-Graduates] 
               WHERE ComandmentNumber = @ComandmentNumber) 
    BEGIN
        SET @StudentNumber += 1

        INSERT INTO dbo.[Defeats-Graduates] (ComandmentNumber, StudentNumber, StudentFakNumber) 
        VALUES (@ComandmentNumber, @StudentNumber, @StudentFakNumber)
    END
    ELSE
    BEGIN
        SET @StudentNumber = 1

        INSERT INTO dbo.[Defeats-Graduates](ComandmentNumber, StudentNumber, StudentFakNumber) 
        VALUES (@ComandmentNumber, @StudentNumber, @StudentFakNumber)
    END
END

但是当我尝试执行此操作时，出现此错误：

无法将值NULL插入表'Test.dbo.Defeats-Graduates'的'StudentNumber'列中；列不允许为空。插入失败。

Answer 1

在第一个条件下，您没有@StudentNumber的赋值语句，您可以尝试以下操作：

CREATE PROCEDURE InsertIntoDefeatsGraduates
    @ComandmentNumber VARCHAR(100),
    @StudentFakNumber INT
AS
BEGIN
    DECLARE @StudentNumber INT 
    SELECT @StudentNumber = StudentNumber  FROM dbo.[Defeats-Graduates] 
               WHERE ComandmentNumber = @ComandmentNumber
    IF ISNULL(@StudentNumber, 0) > 0
      BEGIN
        SET @StudentNumber += 1

        INSERT INTO dbo.[Defeats-Graduates] (ComandmentNumber, StudentNumber, StudentFakNumber) 
        VALUES (@ComandmentNumber, @StudentNumber, @StudentFakNumber)
      END
    ELSE
      BEGIN
        SET @StudentNumber = 1

        INSERT INTO dbo.[Defeats-Graduates](ComandmentNumber, StudentNumber, StudentFakNumber) 
        VALUES (@ComandmentNumber, @StudentNumber, @StudentFakNumber)
      END
    END

Answer 2

您从不初始化您的@StudentNumber-您只需声明它-然后为NULL-并且将+1添加到NULL仍然为NULL-因此，最后，您尝试将NULL插入表格中

-- at this point, @StudentNumber is NULL 
DECLARE @StudentNumber INT 

IF EXISTS (SELECT * FROM dbo.[Defeats-Graduates] 
           WHERE ComandmentNumber = @ComandmentNumber) 
BEGIN
    -- trying to increment a NULL value still results in NULL
    SET @StudentNumber += 1

    -- here you're about to insert NULL into "StudentNumber"
    INSERT INTO dbo.[Defeats-Graduates] (ComandmentNumber, StudentNumber, StudentFakNumber) 
    VALUES (@ComandmentNumber, @StudentNumber, @StudentFakNumber)

如果存在，您需要做的是读取实际值：

IF EXISTS (SELECT * FROM dbo.[Defeats-Graduates] 
           WHERE ComandmentNumber = @ComandmentNumber) 
BEGIN
    -- get the highest value and increment by one
    SELECT @StudentNumber = MAX(StudentNumber) + 1
    FROM dbo.[Defeats-Graduates] 
    WHERE ComandmentNumber = @ComandmentNumber;

BE AWARE：如果您有一个繁忙的系统且有多个客户端同时工作并插入数据，则SELECT MAX()+1方法很容易产生重复的值。只要有可能，就应该使用适当的database-provided机制-类似于SQL Server中的SEQUENCE或类似的东西，其设计和构建即使在繁重的工作量下也可以提供唯一的值。