Fehler in FD-Erhaltung Trigger in SQL Server

Question

Betrachten Sie R (A, B, C) mit der funktionalen Abhängigkeit B -> C. Ich schrieb den folgenden Trigger, um die FD zu bewahren.

create trigger t1 on R after insert 
as 
if exists(select B from R 
group by B 
having count(distinct C)>1 
) 
print 'B->C FD does not permit!' 
rollback; 

Es funktioniert gut, aber gibt einen Fehler:

The transaction ended in the trigger. The batch has been aborted.

Wie kann ich fangen/diesen Fehler beheben?

Answer 1

können Sie INSTEAD OF Trigger verwenden:

CREATE TRIGGER t1 on R 
INSTEAD OF INSERT 
AS 
BEGIN 
    IF EXISTS(
     SELECT B 
     FROM (
     SELECT B, C 
     FROM R 
     UNION ALL 
     SELECT B, C 
     FROM INSERTED) AS T 
     GROUP BY B 
     HAVING COUNT(DISTINCT C) > 1 
    ) 
    PRINT 'B->C FD does not permit!' 
    ELSE 
    INSERT INTO R SELECT * FROM inserted 
END; 
GO 

Ein anderer Weg:

CREATE TRIGGER t1 on R 
AFTER INSERT 
AS 
     IF EXISTS(
      SELECT B 
      FROM (
      SELECT B, C 
      FROM R 
      UNION ALL 
      SELECT B, C 
      FROM INSERTED) AS T 
      GROUP BY B 
      HAVING COUNT(DISTINCT C) > 1 
     ) 
BEGIN 
PRINT 'B->C FD does not permit!' 
ROLLBACK TRANSACTION; 
RETURN 
END; 
GO 

Answer 2

Die Abfrage in Ihrem Trigger ist in keinem Zusammenhang zu dem, was Sie eingefügt haben, die inserted Tabelle ist.

create trigger t1 on R after insert 
as 
if exists(select B from R 
where B in (select B from inserted) --"is" was mistype 
group by B 
having count(C)>1 --distinct allows to insert same (B,C) pairs 
) 
begin 
print 'B->C FD does not permit!' 
rollback; 
end 

-- insert fresh data 
insert R (A,B,C) 
values 
(1,'ab','cd'), 
(2,'cd','ef') 
--passed 
-- insert more 
insert R (A,B,C) 
values 
(3,'abc','cd'), 
(4,'cde','ef'), 
(5,'ab','qq') 
-- failed with message: 
B->C FD does not permit! 
Msg 3609, Level 16, State 1, Line 16 
The transaction ended in the trigger. The batch has been aborted. 
-- As Expected