创建一个在 PL/SQL 中自动分配成绩的触发器

Question

I want to make a trigger that when I update student's mark in a table.. then it will automatically assign a grade.

CREATE OR ALTER TRIGGER calculate_grade
AFTER INSERT OR UPDATE ON student_subject
BEGIN
    UPDATE student_subject SET grade =
    CASE
        WHEN (new.firstterm+new.secondterm) >= 80 THEN 'A'
        WHEN (new.firstterm+new.secondterm) >= 65 AND (new.firstterm+new.secondterm) < 80 THEN 'B'
        WHEN (new.firstterm+new.secondterm) >= 50 AND (new.firstterm+new.secondterm) < 65 THEN 'C'
        WHEN (new.firstterm+new.secondterm) >= 40 AND (new.firstterm+new.secondterm) < 50 THEN 'D'
        WHEN (new.firstterm+new.secondterm) >= 20 AND (new.firstterm+new.secondterm) < 40 THEN 'E'
        ELSE 'F'
    END
END

but it found it errors.. how do I fix it

Answer 1

For this use case I would recommend using virtual column . You will save on space (only metadata are stored about the column) and grade will be always in sync (someone can disable trigger for a while and this could cause problems with data integrity using trigger based approach). DML on the table will be also faster.

create table t(
  id number primary key,
  firstterm number,
  secondterm number,
  grade char(1) generated always as (
        case
            when firstterm + secondterm >= 80 then 'A'
            when firstterm + secondterm >= 65 then 'B'
            when firstterm + secondterm >= 50 then 'C'
            when firstterm + secondterm >= 40 then 'D'
            when firstterm + secondterm >= 20 then 'E'
            when firstterm + secondterm >= 0  then 'F'
            else null
        end
  ) virtual
)

sqlfiddle

Answer 2

You cannot Update the table that fired the trigger, you are already in the process of updating (or inserting) it so I'm guessing you getting a "ORA-04091: table is mutating" exception. If not you will. You could be getting compile errors. But updating is not necessary. Instead of attempting update just use assignment. Also references to New and/or Old require a leading colon (:).

create or alter trigger calculate_grade
after insert or update on student_subject
begin
    :new.grade :=
        case
            when (:new.firstterm+:new.secondterm) >= 80 then 'A'
            when (:new.firstterm+:new.secondterm) >= 65 and (:new.firstterm+:new.secondterm) < 80 then 'B';
            when (:new.firstterm+:new.secondterm) >= 50 and (:new.firstterm+:new.secondterm) < 65 then 'C';
            when (:new.firstterm+:new.secondterm) >= 40 and (:new.firstterm+:new.secondterm) < 50 then 'D';
            when (:new.firstterm+:new.secondterm) >= 20 and (:new.firstterm+:new.secondterm) < 40 then 'E';
            else 'F'
        end;
end ;

Answer 3

I would suggest a before trigger:

CREATE OR ALTER TRIGGER calculate_grade
BEFORE INSERT OR UPDATE ON student_subject
BEGIN
    SELECT (CASE WHEN (:new.firstterm + :new.secondterm) >= 80 THEN 'A'
                 WHEN (:new.firstterm + :new.secondterm) >= 65 THEN 'B'
                 WHEN (:new.firstterm + :new.secondterm) >= 50 THEN 'C'
                 WHEN (:new.firstterm + :new.secondterm) >= 40 THEN 'D'
                 WHEN (:new.firstterm + :new.secondterm) >= 20 THEN 'E'
                 ELSE 'F'
            END)
    INTO :NEW.GRADE
    FROM DUAL;
END;

Note that the CASE logic is evaluated in order, so you don't have to use BETWEEN for the conditions.