Question

I’m studying SQL and can’t seem to find an answer to this exercise.

Exercise: For all cases where the same reviewer rated the same movie twice and gave it a higher rating the second time, return the reviewer’s name and the title of the movie.

I don’t know how to compare 2 rows and then get the higher rating.

The tables’ schemas are:

• Movie ( mID, title, year, director )
  English: There is a movie with ID number mID, a title, a release
  year, and a director.
• Reviewer ( rID, name )
  English: The reviewer with ID number rID has a certain name.
• Rating ( rID, mID, stars, ratingDate )
  English: The reviewer rID gave the movie mIDa number of stars rating (1-5) on a certain ratingDate.*

Researching here in the forum I’ve got as far as to this point:

select *
from rating a
join Reviewer rv on rv.rid = a.rid
where 1 < (select COUNT(*) from rating b
            where b.rid = a.rid and b.mid = a.mid)

I’d be glad to be given also an explanation of the code. Since even the code above is making me really confused.

/* Create the schema for our tables */
create table Movie(mID int, title text, year int, director text);
create table Reviewer(rID int, name text);
create table Rating(rID int, mID int, stars int, ratingDate date);

/* Populate the tables with our data */
insert into Movie values(101, 'Gone with the Wind', 1939, 'Victor Fleming');
insert into Movie values(102, 'Star Wars', 1977, 'George Lucas');
insert into Movie values(103, 'The Sound of Music', 1965, 'Robert Wise');
insert into Movie values(104, 'E.T.', 1982, 'Steven Spielberg');
insert into Movie values(105, 'Titanic', 1997, 'James Cameron');
insert into Movie values(106, 'Snow White', 1937, null);
insert into Movie values(107, 'Avatar', 2009, 'James Cameron');
insert into Movie values(108, 'Raiders of the Lost Ark', 1981, 'Steven Spielberg');

insert into Reviewer values(201, 'Sarah Martinez');
insert into Reviewer values(202, 'Daniel Lewis');
insert into Reviewer values(203, 'Brittany Harris');
insert into Reviewer values(204, 'Mike Anderson');
insert into Reviewer values(205, 'Chris Jackson');
insert into Reviewer values(206, 'Elizabeth Thomas');
insert into Reviewer values(207, 'James Cameron');
insert into Reviewer values(208, 'Ashley White');

insert into Rating values(201, 101, 2, '2011-01-22');
insert into Rating values(201, 101, 4, '2011-01-27');
insert into Rating values(202, 106, 4, null);
insert into Rating values(203, 103, 2, '2011-01-20');
insert into Rating values(203, 108, 4, '2011-01-12');
insert into Rating values(203, 108, 2, '2011-01-30');
insert into Rating values(204, 101, 3, '2011-01-09');
insert into Rating values(205, 103, 3, '2011-01-27');
insert into Rating values(205, 104, 2, '2011-01-22');
insert into Rating values(205, 108, 4, null);
insert into Rating values(206, 107, 3, '2011-01-15');
insert into Rating values(206, 106, 5, '2011-01-19');
insert into Rating values(207, 107, 5, '2011-01-20');
insert into Rating values(208, 104, 3, '2011-01-02');

Answer 1

something like that should work (they are other ways, too)

SELECT rev.name, m.title
FROM Reviewer rev
INNER JOIN Rating r1 on r1.rID = rev.rID
INNER JOIN Rating r2 on r2.rID = rev.rID and r2.mID = r1.mID
INNER JOIN Movie m on m.mID = r1.mID
WHERE r2.ratingDate > r1.ratingDate and r2.stars > r1.stars 

or you can do all in join (instead of WHERE clause) in this case

SELECT rev.name, m.title
FROM Reviewer rev
INNER JOIN Rating r1 on r1.rID = rev.rID
INNER JOIN Rating r2 
  on r2.rID = rev.rID 
  and r2.mID = r1.mID
  and r2.ratingDate > r1.ratingDate
  and r2.stars > r1.stars
INNER JOIN Movie m on m.mID = r1.mID

SqlFiddle (with your sample datas)

Explanation : I suppose you know the JOIN syntax, so

The trick is to join Rating two times.
Then the WHERE part checks if there’s exist a line where one of the rating (from same reviewer on same movie) has a bigger ratingDate and more stars. Which checks : “gave it a higher rating the second time”.

Then we just group by reviewerName and movie title (this part is to avoid duplicates if we have 3 reviews, the second having more stars than the first, and the third more than the second) : with your sample datas, the GROUP BY is not needed, but…