Question

I have a table schema as

create table Location(
id int primary key,
city varchar(255),
state varchar(100),
country varchar(255)
);
create table Person( 
id int primary key,
name varchar(100)
);
create table Photographer(
id int primary key references Person(id) on update cascade on delete cascade,
livesIn int not null references Location(id) on update cascade on delete no action
);
create table Specialty(
photographer int references Photographer(id) on update cascade on delete cascade,
type enum('portrait','landscape','sport'),
primary key(photographer, type)
);
create table Photo(
id int primary key,
takenAt timestamp not null,
takenBy int references Photographer(id) on update cascade on delete no action,
photographedAt int references Location(id) on update cascade on delete no action
);
create table Appearance(
  shows int references Person(id) on update cascade on delete cascade,
isShownIn int references Photo(id) on update cascade on delete cascade,
primary key(shows, isShownIn)
);

I am stuck at two queries :

1) The photos such that the photo only shows photographers that live in the same location. List each photo once. That is, photos must have persons that are photographers, and they all need to live in the same place.

2) The locations that have the property that every photo in the location was taken by a photographer who is not shown in any photo in Massachusetts? For each location show only the city, and show each location only once.

My tries :
1)

SELECT ph.id, ph.takenAt, ph.takenBy, ph.photographedAt FROM 
(SELECT * FROM Photo p, Appearance ap WHERE p.id = ap.isShownIn 
HAVING ap.shows IN (SELECT person.id FROM Person,Photographer WHERE person.id
photographer.id)) ph
WHERE ph.photographedAt = (SELECT location.id FROM location WHERE location.id = 
(SELECT livesIn FROM Photographer WHERE id = ph.takenBy))

2)

select distinct city from location where location.id in (
select photographedAt from photo, (select * from appearance where appearance.shows in
(select photographer.id from photographer)) ph
where photo.id = ph.isShownIn )
and location.state <> 'Massachusetts'

Can anyone help in creating these queries ??

Answer 1

Your queries are both of the “list individual items that have properties X and Y, where X and Y are in different tables” variety.

These types of questions are commonly solved using correlated sub-queries with EXISTS and NOT EXISTS.

Using EXISTS takes care of the “show each item only once” part. Otherwise you would need to use grouping in conjunction with complex joins, and this can get messy very quickly.

Question 1 requires:

[…] photos must have persons that are photographers, and they all need to live in the same place.

Note that this definition doesn’t say “do not show photos if they contain other people, too”. If that’s what you really meant, it’s upon you to draw conclusions from the SQL below and to write better definitions next time. 😉

SELECT
  *
FROM
  Photo p
WHERE
  EXISTS (
    -- ...that has at least one appearance of a photographer
    SELECT 
      1 
    FROM
      Appearance              a 
      INNER JOIN Photographer r ON r.id = a.shows
      INNER JOIN Location     l ON l.id = r.livesIn
    WHERE
      a.isShownIn = p.id
      -- AND l.id = <optional location filter would go here>
      AND NOT EXISTS (
        -- ...that does not have an appearance of a photographer from 
        --    some place else
        SELECT 
          1 
        FROM
          Appearance              a1 
          INNER JOIN Photographer r1 ON r1.id = a1.shows
          INNER JOIN Location     l1 ON l1.id = r1.livesIn
        WHERE
          a1.isShownIn = p.Id
          AND l1.id <> l.id
      )
  )

The second question reads

[…] locations that have the property that every photo in the location was taken by a photographer who is not shown in any photo in Massachusetts? For each location show only the city, and show each location only once.

The according SQL would look like:

SELECT
  city
FROM
  Location l
WHERE
  NOT EXISTS (
    -- ...a photo at this location taken by a photographer who makes 
    --    an apperance on another photo which which was taken in Massachusetts
    SELECT
      1
    FROM
      Photo                    p
      INNER JOIN Photographer  r ON r.id = p.takenBy
      INNER JOIN Appearance    a ON a.shows = r.id
      INNER JOIN Photo        p1 ON p1.id = a.isShownIn
    WHERE
      p.photographedAt = l.Id
      AND p1.photographedAt = <the location id of Massachusetts>
  )