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Unformatted text preview: More SQL
Extended Relational Algebra
Outerjoins, Grouping/Aggregation
Insert/Delete/Update
1 The Extended Algebra
δ = eliminate duplicates from bags.
τ = sort tuples.
γ = grouping and aggregation.
Outerjoin : avoids “dangling tuples” = tuples that do not join with anything. 2 Duplicate Elimination
x R1 := δ(R2).
x R1 consists of one copy of each tuple that appears in R2 one or more times. 3 Example: Duplicate Elimination
R = ( A
1
3
1 B )
2
4
2 δ(R) = A
1
3 B
2
4 4 Sorting
x R1 := τL (R2). L is a list of some of the attributes of R2. x R1 is the list of tuples of R2 sorted first on the value of the first attribute on L, then on the second attribute of L, and so on.
Break ties arbitrarily. xτ is the only operator whose result is neither a set nor a bag. 5 Example: Sorting
R = ( A
1
3
5 B )
2
4
2 τB (R) = [(5,2), (1,2), (3,4)] 6 Aggregation Operators
x Aggregation operators are not operators of relational algebra.
x Rather, they apply to entire columns of a table and produce a single result.
x The most important examples: SUM, AVG, COUNT, MIN, and MAX. 7 Example: Aggregation
R = ( A
1
3
3 B )
3
4
2 SUM(A) = 7
COUNT(A) = 3
MAX(B) = 4
AVG(B) = 3
8 Grouping Operator
x R1 := γL (R2). L is a list of elements that are either:
1. Individual (grouping ) attributes.
2. AGG(A ), where AGG is one of the aggregation operators and A is an attribute.
• An arrow and a new attribute name renames the component.
9 Applying γL(R)
x Group R according to all the grouping attributes on list L.
That is: form one group for each distinct list of values for those attributes in R. x Within each group, compute AGG(A ) for each aggregation on list L.
x Result has one tuple for each group:
1. The grouping attributes and
2. Their group’s aggregations. 10 Example: Grouping/Aggregation
R = ( A
1
4
1 B
2
5
2 γA,B,AVG(C)>X (R) = ?? C )
3
6
5 First, group R by A and B :
A
B
C
1
2
3
1
2
5
4
5
6 Then, average C within groups:
A
1
4 B
2
5 X
4
6 11 Outerjoin
x Suppose we join R ⋈C S.
x A tuple of R that has no tuple of S with which it joins is said to be dangling.
Similarly for a tuple of S. x Outerjoin preserves dangling tuples by padding them NULL.
12 Example: Outerjoin
R = ( A
1
4 B )
2
5 S = ( B
2
6 C )
3
7 (1,2) joins with (2,3), but the other two tuples
are dangling.
R OUTERJOIN S = A
1
4
NULL B
2
5
6 C
3
NULL
7
13 Now Back to SQL
Each Operation Has a SQL Equivalent 14 Outerjoins
x R OUTER JOIN S is the core of an outerjoin expression. It is modified by:
1. Optional NATURAL in front of OUTER.
2. Optional ON <condition> after JOIN.
3. Optional LEFT, RIGHT, or FULL before OUTER. Only one
of these x LEFT = pad dangling tuples of R only.
x RIGHT = pad dangling tuples of S only.
x FULL = pad both; this choice is the default.
15 Aggregations
x SUM, AVG, COUNT, MIN, and MAX can be applied to a column in a SELECT clause to produce that aggregation on the column.
x Also, COUNT(*) counts the number of tuples. 16 Example: Aggregation
x From Sells(bar, beer, price), find the average price of Bud:
SELECT AVG(price)
FROM Sells
WHERE beer = ’Bud’; 17 Eliminating Duplicates in an Aggregation
x Use DISTINCT inside an aggregation.
x Example: find the number of different prices charged for Bud:
SELECT COUNT(DISTINCT price)
FROM Sells
WHERE beer = ’Bud’;
18 NULL’s Ignored in Aggregation
x NULL never contributes to a sum, average, or count, and can never be the minimum or maximum of a column.
x But if there are no nonNULL values in a column, then the result of the aggregation is NULL.
Exception: COUNT of an empty set is 0.
19 Example: Effect of NULL’s
SELECT count(*)
FROM Sells
WHERE beer = ’Bud’;
SELECT count(price)
FROM Sells
WHERE beer = ’Bud’; The number of bars
that sell Bud. The number of bars
that sell Bud at a
known price. 20 Grouping
x We may follow a SELECTFROM
WHERE expression by GROUP BY and a list of attributes.
x The relation that results from the SELECTFROMWHERE is grouped according to the values of all those attributes, and any aggregation is applied only within each group.
21 Example: Grouping
x From Sells(bar, beer, price), find the average price for each beer:
SELECT beer, AVG(price)
FROM Sells
GROUP BY beer;
beer
Bud
… AVG(price)
2.33
…
22 Example: Grouping
x From Sells(bar, beer, price) and Frequents(drinker, bar), find for each drinker the average price of Bud at the bars they frequent:
Compute all
SELECT drinker, AVG(price)
drinkerbar
price triples
FROM Frequents, Sells
for Bud.
WHERE beer = ’Bud’ AND
Then group
Frequents.bar = Sells.bar them by
GROUP BY drinker;
drinker.
23 Restriction on SELECT Lists With Aggregation
x If any aggregation is used, then each element of the SELECT list must be either:
1. Aggregated, or
2. An attribute on the GROUP BY list. 24 Illegal Query Example
x You might think you could find the bar that sells Bud the cheapest by:
SELECT bar, MIN(price)
FROM Sells
WHERE beer = ’Bud’;
x But this query is illegal in SQL.
25 HAVING Clauses
x HAVING <condition> may follow a GROUP BY clause.
x If so, the condition applies to each group, and groups not satisfying the condition are eliminated. 26 Example: HAVING
x From Sells(bar, beer, price) and Beers(name, manf), find the average price of those beers that are either served in at least three bars or are manufactured by Pete’s. 27 Solution
Beer groups with at least
3 nonNULL bars and also
beer groups where the
manufacturer is Pete’s. SELECT beer, AVG(price)
FROM Sells
GROUP BY beer
HAVING COUNT(bar) >= 3 OR
beer IN (SELECT name FROM Beers WHERE manf = ’Pete’’s’);
28 Beers manu
factured by
Pete’s. Requirements on HAVING Conditions
x Anything goes in a subquery.
x Outside subqueries, they may refer to attributes only if they are either:
1. A grouping attribute, or
2. Aggregated (same condition as for SELECT clauses with aggregation).
29 Database Modifications
x A modification command does not return a result (as a query does), but changes the database in some way.
x Three kinds of modifications: 1. Insert a tuple or tuples.
2. Delete a tuple or tuples.
3. Update the value(s) of an existing tuple or tuples.
30 Insertion
x To insert a single tuple: INSERT INTO <relation>
VALUES ( <list of values> );
x Example: add to Likes(drinker, beer) the fact that Sally likes Bud.
INSERT INTO Likes
VALUES(’Sally’, ’Bud’);
31 Specifying Attributes in INSERT
x We may add to the relation name a list of attributes.
x Two reasons to do so: 1. We forget the standard order of attributes for the relation.
2. We don’t have values for all attributes, and we want the system to fill in missing components with NULL or a default value.
32 Example: Specifying Attributes
x Another way to add the fact that Sally likes Bud to Likes(drinker, beer):
INSERT INTO Likes(beer, drinker)
VALUES(’Bud’, ’Sally’); 33 Adding Default Values
x In a CREATE TABLE statement, we can follow an attribute by DEFAULT and a value.
x When an inserted tuple has no value for that attribute, the default will beused. 34 Example: Default Values
CREATE TABLE Drinkers (
name CHAR(30) PRIMARY KEY,
addr CHAR(50)
DEFAULT ’123 Sesame St.’,
phone CHAR(16)
);
35 Example: Default Values
INSERT INTO Drinkers(name)
VALUES(’Sally’);
Resulting tuple:
name
address
Sally
123 Sesame St phone
NULL 36 Inserting Many Tuples
x We may insert the entire result of a query into a relation, using the form:
INSERT INTO <relation>
( <subquery> ); 37 Example: Insert a Subquery
x Using Frequents(drinker, bar), enter into the new relation PotBuddies(name) all of Sally’s “potential buddies,” i.e., those drinkers who frequent at least one bar that Sally also frequents. 38 The other
drinker Solution Pairs of Drinker
tuples where the
first is for Sally,
the second is for
someone else,
and the bars are
the same. INSERT INTO PotBuddies
(SELECT d2.drinker FROM Frequents d1, Frequents d2 WHERE d1.drinker = ’Sally’ AND
d2.drinker <> ’Sally’ AND
d1.bar = d2.bar
);
39 Deletion
x To delete tuples satisfying a condition from some relation:
DELETE FROM <relation>
WHERE <condition>; 40 Example: Deletion
x Delete from Likes(drinker, beer) the fact that Sally likes Bud:
DELETE FROM Likes
WHERE drinker = ’Sally’ AND
beer = ’Bud’; 41 Example: Delete all Tuples
x Make the relation Likes empty:
DELETE FROM Likes;
x Note no WHERE clause needed. 42 Example: Delete Some Tuples
x Delete from Beers(name, manf) all beers for which there is another beer by the same manufacturer.
Beers with the same
manufacturer and
DELETE FROM Beers b
a different name
from the name of
WHERE EXISTS (
the beer represented
SELECT name FROM Beers by tuple b.
WHERE manf = b.manf AND
name <> b.name);
43 Semantics of Deletion (1)
x Suppose AnheuserBusch makes only Bud and Bud Lite.
x Suppose we come to the tuple b for Bud first.
x The subquery is nonempty, because of the Bud Lite tuple, so we delete Bud.
x Now, when b is the tuple for Bud Lite, do we delete that tuple too?
44 Semantics of Deletion (2)
x Answer: we do delete Bud Lite as well.
x The reason is that deletion proceeds in two stages:
1. Mark all tuples for which the WHERE condition is satisfied.
2. Delete the marked tuples. 45 Updates
x To change certain attributes in certain tuples of a relation:
UPDATE <relation>
SET <list of attribute assignments>
WHERE <condition on tuples>; 46 Example: Update
x Change drinker Fred’s phone number to 5551212:
UPDATE Drinkers
SET phone = ’5551212’
WHERE name = ’Fred’;
47 Example: Update Several Tuples
x Make $4 the maximum price for beer:
UPDATE Sells
SET price = 4.00
WHERE price > 4.00; 48 ...
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 Fall '99
 Widom
 Databases, Sort

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