SQL: Queries, Constraints, Triggers
SQL Division
CS430/630
Lecture 7
Slides based on “Database Management Systems” 3rd ed, Ramakrishnan and Gehrke
Division
Used to answer queries such as:
Find sailors who have reserved all boats.
Let A have 2 fields, x and y; B have only field y:
A/B =
A/B contains all x tuples (sailors) such that for every y tuple
(boat) in B, there is an xy tuple in A
Or, if the set of y values (boats) associated with an x value (sailor)
in A contains all y values in B, the x value is in A/B.
In general, x and y can be any sets of fields (not singletons)
x x y A y B| ,
bid
b1
b2
b4
bid
b2
b4
bid
b2
sid
s1
s2
s3
s4
sid
s1
s4
sid
s1
Examples of Division A/B
A
B1
B2
B3
A/B1 A/B2 A/B3
sid bid
s1 b1
s1 b2
s1 b3
s1 b4
s2 b1
s2 b2
s3 b2
s4 b2
s4 b4
Query 1
“Find the names of sailors who’ve reserved all boats”
( , (
,
Re ) / ( ))Tempsids
sid bid
serves
bid
Boats
sname Tempsids Sailors( )
Query 2
“Find sailors who’ve reserved all red boats”
)))
”
((/)Re
,
(,( Boats
redcolorbid
serves
bidsid
Temp
)( SailorsTempsname
Expressing A/B Using Basic Operators
For A/B, compute all x values that are not disqualified by
some y value in B
x value is disqualified if by attaching y value from B, we obtain an
xy tuple that is not in A
Disqualified x values:
A/B:
)))((( ABAxx
x A( ) all disqualified tuples
)))((()( ABAxxAx
Division in SQL
Not supported as primitive operator
Need to use nested queries to express division
One of the most subtle queries in SQL
Need to pay close attention to writing SQL division queries!
There are two ways of writing division queries
Using the set EXCEPT operator (2-level nesting)
Without the EXCEPT operator (3-level nesting)
Division: Solution 1
With EXCEPT:
SELECTS.sname
FROMSailors S
WHERENOT EXISTS
(
(SELECTB.bid FROMBoats B)
EXCEPT
(SELECTR.bid FROMReserves R
WHERER.sid=S.sid)
)
“Find sailors who’ve reserved all boats.”
Division: Solution 2
SELECTS.sname
FROMSailors S
WHERENOT EXISTS(SELECTB.bid
FROMBoats B
WHERENOT EXISTS(SELECT*
FROMReserves R
WHERER.bid=B.bid
AND R.sid=S.sid))
Sailors S such that …
there is no boat B …
without a Reserves tuple showing S reserved B
“Find sailors who’ve reserved all boats.”
Without EXCEPT:
“Find sailors who’ve reserved all red boats.”
With EXCEPT:
SELECTS.sname
FROMSailors S
WHERENOT EXISTS
(
(SELECTB.bid FROMBoats B
WHERE B.color = ‘red’)
EXCEPT
(SELECTR.bid FROMReserves R
WHERER.sid=S.sid) )
“Find sailors who’ve reserved all red boats.”
Without EXCEPT:
“Find sailors who’ve reserved all red boats.”
SELECTS.sname
FROMSailors S
WHERENOT EXISTS(SELECTB.bid
FROMBoats B
WHEREB.color=‘red’ AND
NOT EXISTS(SELECT*
FROMReserves R
WHERER.bid=B.bid
AND R.sid=S.sid))
“Find sailors who’ve reserved all red boats.”
Another Example
SELECT A.name FROM Actors A
WHERE NOT EXISTS(
SELECT M.movie_id FROM Movies M
WHERE M.year=1990 AND M.studio=‘Universal’
EXCEPT
SELECT S.movie_id FROM Stars_In S
WHERE S.actor_id=A.actor_id
)
“Find names of actors who star in ALL movies produced by Universal in year 1990.”
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