SQL:@somevalue = null无法正常工作

Question

我试过@somevalue = null并且它工作了.但现在它不起作用.如何检查值是否为空？简而言之,我需要做的是,如果@existPerson等于null,则将值插入数据库.但如果它存在则返回-2.我已经检查了数据库,确保没有与@nic = 100相同的值.即便如此,它返回我-2.可能的原因是什么？非常感谢!

CREATE PROCEDURE [dbo].[addStudent] 
    @stuName varchar(50), 
    @address varchar(100),
    @tel varchar(15),
    @etel varchar(15),
    @nic varchar (10),
    @dob date


AS 
BEGIN   
    SET NOCOUNT ON;

    DECLARE @currentID INT
    DECLARE @existPerson INT
    SET @existPerson = (SELECT p_ID FROM Student WHERE s_NIC = @nic);
    IF @existPerson = null
        BEGIN
            INSERT INTO Person (p_Name, p_RegDate, p_Address, p_Tel, p_EmergeNo, p_Valid, p_Userlevel)
            VALUES (@stuName,  GETDATE(), @address, @tel, @etel, 0, 'Student' );
            SET @currentID = (SELECT MAX( p_ID) FROM Person); 
            INSERT INTO Student (p_ID, s_Barcode, s_DOB, s_NIC) VALUES (@currentID , NULL, @dob, @nic);
            return 0;
        END
    ELSE
        return -2;
END 

Answer 1

这应该是

  IF @existPerson is null

在sql中,如果要检查值是否为null,则需要使用is null或is not null代替= null或<> null