Download Subqueries and Joining

SQL queries
subqueries and joining
SQL query – beyond one table
• So far, we have only applied queries to a
single table
• It is possible – and very useful – to make
queries applied to several tables
• Can answer more complex questions…
• …but queries also tend to become more
complex themselves 
SWC – 2011
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SQL query – beyond one table
• We use a movie information database
as example
Movie
movieid
title
country
prodyear
genre
oscars
Actor
Casting
movieid
actorid
SWC – 2011
actorid
name
country
birth
living
oscars
3
SQL query – beyond one table
actorid
name
country
birth
1
John Wayne
USA
04-06-1911 No
2
2
Chow-Yun Fat
HK
12-11-1959 Yes
0
3
Karl Stegger
Denmark
30-01-1922 No
0
4
Jean Reno
France
10-10-1949 Yes
0
5
Julia Roberts
USA
28-09-1964 Yes
1
6
Natalie Portman
Israel
02-07-1980 Yes
0
7
Marc Duret
France
01-08-1966 Yes
1
SWC – 2011
living
oscars
4
SQL query – beyond one table
movieid
actorid
movieid
actorid
1
1
4
4
1
4
4
6
1
5
5
2
1
7
5
4
2
4
5
6
2
6
6
5
2
7
6
6
3
1
7
2
3
2
7
4
3
3
7
5
SWC – 2011
5
SQL query – subqueries
• Suppose we wish to answer a (complex)
question like this:
• How many movies are from the country
that Marc Duret is from
• This question cannot be answered by a
single query (as we know it…)
• Information is spread out in multiple tables
SWC – 2011
6
SQL query – subqueries
SELECT country
FROM Actor
WHERE name = ’Marc Duret’
Result is
’France’
SELECT COUNT(title) AS MovieCount
FROM Movie
WHERE Movie.country = ’France’
SWC – 2011
’France’
used as
input
7
SQL query – subqueries
• We can run these two
queries ”manually”, or
use output from first
query as input to second
query – not optimal…
• Better to make first
query a subquery to the
second query
SWC – 2011
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SQL query – subqueries
Outer
query
SELECT COUNT(title) AS MovieCount
FROM Movie
WHERE country = (SELECT country
FROM Actor
WHERE name = ’Marc Duret’)
Inner
query
SWC – 2011
9
SQL query – subqueries
• Result from inner query (the subquery) is
used as input to outer query
• The inner query produces a result table –
just as any other query – which is then
used by the outer query
• Very convenient – but some complications
– Name clash
– Multiple results
SWC – 2011
10
SQL query – subqueries
• When we query more than one table, field
names from different tables might be
identical – a name clash
• We can qualify a field name by prefixing it
with the table name
– Movie.country (country field in Movie table)
– Actor.country (country field in Actor table)
SWC – 2011
11
SQL query – subqueries
• What if a subquery produces more than
one result?
SELECT country
FROM Actor
WHERE (oscars > 0)
SWC – 2011
Result:
USA
France
12
SQL query – subqueries
• In such case, we must use the IN keyword
SELECT COUNT(title) AS MovieCount
FROM Movie
WHERE country IN
(SELECT country
FROM Actor
WHERE (oscars > 0))
Try formulating
this query in
plain English…
SWC – 2011
13
SQL query – joining
• Another approach to multi-table queries is
to use join
• To ”join” tables is to make a kind of
”multiplication” of the tables
• Simple case: Join without any conditions:
SELECT *
FROM Movie, Actor
SWC – 2011
14
SQL query – joining
• If the previous query is run against the
MovieInformation database, we get a
result with 49 rows…
• 49 = 7 x 7. All combinations of Movie
records (7) and Actor records (7)
• All fields from both tables are included
• This is rarely what we want…
SWC – 2011
15
SQL query – joining
• In almost all cases, we wish to pick out
those rows where certain fields match
• Example: For each movie, find the title of
the movie, and the names of the actors
appearing in the movie
• We already have this information in the
Casting table (20 records), but only as
identifiers…
SWC – 2011
16
SQL query – joining
• We clearly need information from all three
tables in the MovieInformation database,
so first attempt at a query could be:
SELECT *
FROM Movie, Casting, Actor
• Result: 980 records with a lot of fields…
SWC – 2011
17
SQL query – joining
• First of all, we only need the Title field
from the Movie table, and the Name field
from the Actor table
SELECT Movie.Title, Actor.Name
FROM Movie, Casting, Actor
• Result: 980 records with correct fields
SWC – 2011
18
SQL query – joining
• Next, we only need those records where
the relevant information match
• We are trying to ”substitute” identifiers in
the Casting table with real data (titles and
names)
• In order to get the right titles and names,
the identifiers for this data must match!
SWC – 2011
19
SQL query – joining
• Match identifier for titles and names:
SELECT Movie.Title, Actor.Name
FROM Movie, Casting, Actor
WHERE ((Movie.MovieId = Casting.MovieId)
AND (Actor.ActorId = Casting.ActorId))
• Result: 20 records with correct fields!
SWC – 2011
20
SQL query – joining
• This is a quite common ”pattern” for multitable queries
– Tables representing relations only contain
identifiers (keys in other tables)
– ”Real data” is contained in tables representing
entities (identifiers are keys)
– Obtaining ”real data” for relations requires use
of join, with matching of key fields
SWC – 2011
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