CS 186, Spring 2007, Lecture 8R&G, Chapter 5

Mary Roth

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∃

The important thing is not tostop questioning.

Albert Einstein

Life is just a bowl of queries.

-Anon(not Forrest Gump)

SQL 2 – The Sequel

Review: SQL DML

• DML includes 4 main statements:SELECT (query), INSERT, UPDATE and DELETE

• e.g: SELECT S.name, E.cidFROM Students S, Enrolled EWHERE S.sid = E.sid AND

S.age=19 SELECT

PROJECT

JOIN

SELECT R.sidFROM Boats B,Reserves RWHERE B.color = ‘red’

AND R.bid=B.bid

INTERSECT

SELECT R.sidFROM Boats B,Reserves RWHERE B.color = ‘green’ ANDR.bid=B.bid Reserves

9/131032

9/131051

9/121011

daybidsid

redSanta Maria103

greenTitanic105

bluePinta102

redNina101

colorbnamebid

Boats

2

1

sid

1

sid∩ = 1

sid

Now let’s do this witha self-join

Example: Find sailors who have reserved a redand a green boat

SELECT R1.sidFROM Boats B1, Reserves R1, Boats B2, Reserves R2

WHERE B1.color = ‘red’ AND B1.bid=R1.bid

AND B2.color = ‘green’ AND B2.bid=R2.bid

AND R1.sid=R2.sid

Find sids of sailors who’ve reserved a red and a greenboat

9/131032

9/131051

9/121011

daybidsid

redSanta Maria103

greenTitanic105

bluePinta102

redNina101

colorbnamebid

Find red reserved boats

Find green reserved boats

Find matching green and redreserved boats

2

1

sid

1

sid

= 1

sid

• The WHERE clause can itself contain an SQL query!(Actually, so can FROM and HAVING clauses can too)

Nested Queries

SELECT S.snameFROM Sailors SWHERE S.sid IN (SELECT R.sid FROM Reserves R

WHERE R.bid=103)

e.g. Find the names of sailors who’ve reserved boat #103:

…and then check each the sid ofeach tuple in Sailors to see if it isin S1

First compute the set of all sailors that havereserved boat 103…

9/131032

9/131051

9/121011

daybidsid

278Sam3

392Bilbo2

227Frodo1

ageratingsnamesid

Sailors

Reserves

S

2

sid

S

S

Bilbo

sname

• This nested query was uncorrelated because the subquerydoes not refer to anything in the enclosing query

Nested Queries

SELECT S.snameFROM Sailors SWHERE S.sid IN (SELECT R.sid FROM Reserves R

WHERE R.bid=103)

It can evaluated once and then checked foreach tuple in enclosing query

• Nested queries can also be correlated; the subqueryrefers to the enclosing query

Nested Queries with correlation

The subquery must be re-evaluated for each tuple inenclosing query

SELECT S.snameFROM Sailors SWHERE EXISTS (SELECT * FROM Reserves R WHERE R.bid=103 AND R.sid=S.sid)

EXISTS is a set operator that is true if result of setexpression has at least one tuple

Nested Queries

e.g. Find the names of sailors who’ve reserved boat #103:

9/131032

9/131051

9/121011

daybidsid

278Sam3

392Bilbo2

227Frodo1

ageratingsnamesid

SailorsReserves

S

S

S

SELECT S.snameFROM Sailors SWHERE EXISTS (SELECT * FROM Reserves R WHERE R.bid=103 AND R.sid=S.sid)

daybidsid

Notice that this querycomputes the sameanswer as the previousquery!

321

2 103 9/13

Set-Comparison Operators

• <tuple expression> IN <set expression>– True if <tuple> is a member of <set>– Also, NOT IN

• EXISTS <set expression>– True if <set expression> evaluates to a set with at least one member– Also, NOT EXISTS

• UNIQUE <set expression>– True if <set expression> evaluates to a set with no duplicates; each

row can appear exactly once– Also, NOT UNIQUE

• <tuple expression> comparison op ANY <set expression>– True if <set expression> contains at least one member that makes the

comparison true– Also, op ALL

Use NOT Exists for Division

SELECT S.snameFROM Sailors SWHERE NOT EXISTS (SELECT B.bid FROM Boats B WHERE NOT EXISTS (SELECT R.bid FROM Reserves R WHERE R.bid=B.bid AND R.sid=S.sid))

Find Sailors S such that ...

there is no boat B...

without a reservation bySailor S

Find sailors who’ve reserved all boats.X = set of sailors and Y = set of all boats with reservations.

Recall: X/Y means only give me X tuples that have a match in Y.

2

Division

9/141033

9/121013

9/131032

9/131031

9/121031

daybidsid

278Sam3

392Bilbo2

227Frodo1

ageratingsnamesid

Sailors

Reserves

S

S

S

SELECT S.snameFROM Sailors SWHERE NOT EXISTS (SELECT B.bid FROM Boats B WHERE NOT EXISTS (SELECT R.bid FROM Reserves R WHERE R.bid=B.bid AND R.sid=S.sid))

bluePinta103

redNina101

colorbnamebid

Boats

1101

R

RR

B

B

3103

RR

UNIQUE

9/131032

9/131031

9/121031

daybidsid

278Sam3

392Bilbo2

227Frodo1

ageratingsnamesid

SailorsReserves

S

S

S

daybidsid

2 103 9/13

Find the names of sailors who’ve reserved boat #103exactly once

SELECT S.snameFROM Sailors SWHERE UNIQUE (SELECT sid, bid FROM Reserves R WHERE R.bid=103 AND S.sid=R.sid) 321

ANY

SELECT *FROM Sailors SWHERE S.rating > ANY(SELECT S2.rating FROM Sailors S2 WHERE S2.sname=‘Bilbo’)

Find sailors whose rating is greater than that of some sailorcalled Bilbo:

278Sam3

392Bilbo2

227Frodo1

ageratingsnamesid

S1

278Sam3

392Bilbo2

227Frodo1

ageratingsnamesid

S2

392Bilbo2

ageratingsnamesid

Correlated oruncorrelated?

Uncorrelated!

Intermission

Aggregate Operators• Very powerful; enables computations over sets of tuples

SELECT AVG (S.age)FROM Sailors SWHERE S.rating=10

SELECT COUNT (*)FROM Sailors S

• COUNT: returns a count of tuples inthe set

• AVG: returns average of columnvalues in the set

• SUM: returns sum of column values inthe set

• MIN, MAX: returns min (max) value ofcolumn values in a set.

• DISTINCT can be added to COUNT,AVG, SUM to perform computationonly over distinct values.

SELECT AVG(DISTINCT S.age)FROM Sailors SWHERE S.rating=10

Find name and age of the oldest sailor(s)

SELECT S.sname, MAX (S.age)FROM Sailors S

Aggregate Operators

What will the result be? 278Sam3

392Bilbo2

227Frodo1

ageratingsnamesid

Sailors

39Sam

39Bilbo

39Frodo

agesname

XNot legal syntax; no other columnsallowed in SELECT clause without aGROUP BY clause(we’ll learn about those next)

Find name and age of the oldest sailor(s)

SELECT S.sname, S.ageFROM Sailors SWHERE S.age = (SELECT MAX (S2.age) FROM Sailors S2)

Aggregate Operators

Instead:

Find the maximum age…

And then find thesailors(s) of that age…

GROUP BY and HAVING

• So far, we’ve seen aggregate operators applied to alltuples.– What if we want to apply ops to each of several groups of tuples?

• Consider: Find the age of the youngest sailor for eachrating level.– In general, we don’t know how many rating levels exist, and what

the rating values for these levels are!– Suppose we know that rating values go from 1 to 10; we can

write 10 queries that look like this (!):

SELECT MIN (S.age)FROM Sailors SWHERE S.rating = i

For i = 1, 2, ... , 10:

Queries With GROUP BY

target-list contains:• list of column names from grouping–list• terms with aggregate operations (e.g., MIN (S.age)).

SELECT [DISTINCT] target-listFROM relation-list[WHERE qualification]GROUP BY grouping-list

• To generate values for a column based on groups ofrows, use aggregate functions in SELECT statements withthe GROUP BY clause

First select these rows…

Then group them by the valuesin these columns…

And finally compute aggregatefunction over each group…

Returning 1 row per group

Group By Example

For each rating, find the age of the youngestsailor with age ≥ 18

SELECT S.rating, MIN (S.age)FROM Sailors SWHERE S.age >= 18GROUP BY S.rating

278Sam3

212Pippin4

178Merry5

392Bilbo2

227Frodo1

ageratingsnamesid

Sailors

227Frodo1Group 1

212Pippin4

392Bilbo2Group 2

278Sam3Group 3

212

278

227

Find the number of reservations for eachred boat.

SELECT B.bid, COUNT(*) AS tot_resFROM Boats B, Reserves RWHERE R.bid=B.bid AND B.color=‘red’GROUP BY B.bid

9/141013

9/131032

9/141014

9/121021

daybidsid

Reserves

redSanta Maria103

bluePinta102

redNina101

colorbnamebid

Boats

9/141013

9/141014

9/131032

1103

2101

Queries With GROUP BY and HAVING

• Use the HAVING clause with the GROUP BY clause to restrictwhich group-rows are returned in the result set

SELECT [DISTINCT] target-listFROM relation-listWHERE qualificationGROUP BY grouping-listHAVING group-qualification

Find the age of the youngest sailor with age ≥18, for each rating with at least 2 such sailors

SELECT S.rating, MIN (S.age)FROM Sailors SWHERE S.age >= 18GROUP BY S.ratingHAVING COUNT (*) > 1

Answer

2 3

Sailors who have reservedall boats

278Sam3

392Bilbo2

227Frodo1

ageratingsnamesid

Sailors

9/101021

9/121022

9/141012

9/131032

9/121021

daybidsid

Reserves

redSanta Maria103

bluePinta102

redNina101

colorbnamebid

Boats

SELECT S.nameFROM Sailors S, reserves RWHERE S.sid = R.sidGROUP BY S.name, S.sidHAVING COUNT(DISTINCT R.bid) = ( Select COUNT (*) FROMBoats)

3

count1021Frodo

Bilbo

Bilbo

Bilbo

Frodo

sname

1012

1022

1032

1021

bidsid

Bilbo

Frodo

sname

101, 102, 1032

102,1021

bidsid

Bilbo

Frodo

sname

32

11

countsid

More about Joins

Explicit join semantics needed unless it is an INNER join(INNER is default)

SELECT (column_list)FROM table_name [INNER | {LEFT |RIGHT | FULL } OUTER] JOIN table_name ON qualification_listWHERE …

Default semantics: Inner Join

Only rows that match search conditions are returned.

SELECT s.sid, s.name, r.bidFROM Sailors s INNER JOIN Reserves rON s.sid = r.sid

Returns only those sailors who have reserved boats

SQL-92 also allows:SELECT s.sid, s.name, r.bidFROM Sailors s NATURAL JOIN Reserves r

“NATURAL” means equi-join for each pair of attributeswith the same name

SELECT s.sid, s.name, r.bidFROM Sailors s INNER JOIN Reserves rON s.sid = r.sid

s.sid s.name r.bid

22 Dustin 101

95 Bob 103

Left Outer Join

Left Outer Join returns all matched rows,plus all unmatched rows from the table on theleft of the join clause

(use nulls in fields of non-matching tuples)

SELECT s.sid, s.name, r.bidFROM Sailors s LEFT OUTER JOIN Reserves rON s.sid = r.sid

Returns all sailors & information on whether theyhave reserved boats

SELECT s.sid, s.name, r.bidFROM Sailors s LEFT OUTER JOIN Reserves rON s.sid = r.sid

s.sid s.name r.bid

22 Dustin 101

95 Bob 103

31 Lubber

Right Outer Join

Right Outer Join returns all matched rows, plusall unmatched rows from the table on the rightof the join clause

SELECT r.sid, b.bid, b.nameFROM Reserves r RIGHT OUTER JOIN Boats bON r.bid = b.bid

Returns all boats & information on which onesare reserved.

SELECT r.sid, b.bid, b.nameFROM Reserves r RIGHT OUTER JOIN Boats bON r.bid = b.bid

r.sid b.bid b.name

22 101 Interlake

102 Interlake

95 103 Clipper

104 Marine

Full Outer Join

Full Outer Join returns all (matched orunmatched) rows from the tables on bothsides of the join clause

SELECT r.sid, b.bid, b.nameFROM Reserves r FULL OUTER JOIN Boats bON r.bid = b.bid

Returns all boats & all information onreservations

SELECT r.sid, b.bid, b.nameFROM Reserves r FULL OUTER JOIN Boats bON r.bid = b.bid

r.sid b.bid b.name

22 101 Interlake

102 Interlake

95 103 Clipper

104 MarineNote: in this case it is the same as the ROJ becausebid is a foreign key in reserves, so all reservations musthave a corresponding tuple in boats.

INSERT

INSERT INTO Boats VALUES ( 105, ‘Clipper’, ‘purple’)INSERT INTO Boats (bid, color) VALUES (99, ‘yellow’)

“bulk insert” from one table to another (must be type compatible):INSERT INTO TEMP(bid)SELECT r.bid FROM Reserves R WHERE r.sid = 22;

“bulk insert” from files (in Postgres)Copy

INSERT [INTO] table_name [(column_list)]VALUES ( value_list)

INSERT [INTO] table_name [(column_list)]<select statement>

DELETE & UPDATE

DELETE FROM Boats WHERE color = ‘red’

DELETE FROM Boats bWHERE b. bid = (SELECT r.bid FROM Reserves R WHERE r.sid = 22)

Can also modify tuples using UPDATE statement.UPDATE BoatsSET Color = “green”WHERE bid = 103;

DELETE [FROM] table_name[WHERE qualification]

Null Values

• Values are sometimes– unknown (e.g., a rating has not been assigned or– inapplicable (e.g., no spouse’s name).– SQL provides a special value null for such situations.

• The presence of null complicates many issues. E.g.:– Special operators needed to check if value is/is not null.– “rating>8” - true or false when rating is null? What about

AND, OR and NOT connectives?– Need a 3-valued logic (true, false and unknown).– Meaning of constructs must be defined carefully. (e.g.,

WHERE clause eliminates rows that don’t evaluate to true.)– New operators (in particular, outer joins) possible/needed.

Null Values – 3 Valued Logic

NULL

F

T

NullFTAND

(null > 0)

(null + 1)

(null = 0)

null AND true

is null

is null

is null

is null

NULL

F

T

NullFTOR

T

F Null

T F

F F

Null

NullF

F

T T

T

T NullNullNull