如何处理SQL Server中多行插入的插入触发器？

Question

I have the following trigger:

CREATE TRIGGER trigger_insert_student_table ON student_table
FOR INSERT
AS
BEGIN
    SET NOCOUNT ON;
    DECLARE @insertedStudentName VARCHAR(256);
    DECLARE @insertedMajor VARCHAR(256);
    DECLARE @insertedGrade VARCHAR(256);


    SET @insertedStudentName = (SELECT STUDENT_NAME FROM inserted);
    SET @insertedMajor = (SELECT MAJOR FROM inserted);
    SET @insertedGrade = (SELECT GRADE FROM inserted);

    IF (@insertedMajor='Economics' AND @insertedGrade = 'A')
    BEGIN
        /* DO SOME ACTION*/
    END

    ELSE IF(@insertedMajor='History' AND @insertedGrade = 'A')
    BEGIN
        /* DO SOME ACTION*/
    END
END

It works fine for single inserts, but when I do multi-row inserts it fails with the following error:

Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

How can I iterate through each inserted row and handle the actions?

Answer 1

Don't iterate. As soon as you find yourself saying "how do iterate" in SQL, you're doing it wrong. Instead of setting these values to variables, think set based operations instead. Think along the lines of "where inserted.Major = 'History' and inserted.Grade = 'A'.

I don't know what the "some action" is that you want to do here, but here's a simple example:

update t
set UpdateDate = getdate(),
    isStarEconomicsStudent = 1
from somePresentationTable a
inner join inserted i
    on a.StudentID = i.StudentID
        and i.Major = 'Economics'
        and i.Grade = 'A'