带条件插入的存储过程

Question

我正在尝试使用If 语句编写一个过程。

这是我的数据库

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

例子：

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

但是当我尝试执行这个时，我得到了这个错误：

无法将值 NULL 插入到列“StudentNumber”、表“Test.dbo.Defeats-Graduates”中；列不允许空值。插入失败。

Answer 1

你永远不会初始化你的 @StudentNumber - 你只是声明它 - 然后它是 NULL - 并且将 +1 添加到 NULL 仍然是 NULL - 所以最后，你'正在尝试将 NULL 插入您的表中

-- 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)

你需要做的是读取实际值，如果存在的话：

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;

注意：SELECT MAX()+1 方法非常容易产生重复值，如果您有一个繁忙的系统，多个客户端同时工作和插入数据.只要有可能，您应该使用适当的 database-provided 机制来代替 - 类似于 SQL Server 中的 SEQUENCE 或类似的东西，其设计和构建旨在提供唯一值，即使在繁重的工作负载下也是如此。

Answer 2

在第一个条件下你没有@StudentNumber 的赋值语句，你可以试试这个：

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