SQL代替触发器和更新

Question

I have these tables:

• Movie ( mID, title, year, director )
• Reviewer ( rID, name )
• Rating ( rID, mID, stars, ratingDate )

and some views:

• View LateRating contains movie ratings after January 20, 2011. The view contains the movie ID, movie title, number of stars, and rating date.

   create view LateRating as select distinct R.mID, title, stars, ratingDate from Rating R, Movie M where R.mID = M.mID and ratingDate > '2011-01-20' 

• View HighlyRated contains movies with at least one rating above 3 stars. The view contains the movie ID and movie title.

   create view HighlyRated as select mID, title from Movie where mID in (select mID from Rating where stars > 3) 

• View NoRating contains movies with no ratings in the database. The view contains the movie ID and movie title.

   create view NoRating as select mID, title from Movie where mID not in (select mID from Rating) 

Here's my data set : https://prod-c2g.s3.amazonaws.com/db/Winter2013/files/viewmovie.sql

I'm asked to write an instead-of trigger that enables updates to the stars attribute of view LateRating.Here's my approach.

CREATE trigger update_LateRating_title INSTEAD OF 
UPDATE OF stars ON LateRating
BEGIN
  UPDATE Rating SET stars = stars - 2 
  WHERE Rating.mID = old.mID 
  AND Rating.mID IN (SELECT stars FROM Rating WHERE stars > 2);
END;

It gives almost right answer but there is only one wrong row that being 201 101 4 2011-01-27.4 should be 2.What is wrong?

Answer 1

Let's look at your WHERE condition:

...AND Rating.mID IN (SELECT stars FROM Rating WHERE stars > 2)

It states that Rating.mID should be among values of stars selected from rating. I'm unsure what was intended here. Let's assume you want to decrease the number of stars by 2 for any attempt to modify stars (that's weird: you're turning queries which would increase stars into some unrelated queries) if the original number of stars in that line of Rating table was greater than 2.

CREATE trigger update_LateRating_stars INSTEAD OF 
UPDATE OF stars ON LateRating
BEGIN
  UPDATE Rating SET stars = stars - 2 
  WHERE Rating.mID = old.mID 
  AND Rating.stars > 2;
END;

Again, there are many possible interpretations of what you want, and they all are more or less weird : mID field in LateRating is not enough to identify the original record in Rating (maybe rID and mID together would be enough, if you don't allow the same reviewer to hold different opinions on the same movie simultaneously).