Introduction to Data Management. Lecture 14 (SQL: the Saga Continues...)

Transcription

1 Introduction to Data Management Lecture 14 (SQL: the Saga Continues...) Instructor: Mike Carey Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 Announcements v HW and exams: HW #4 info Still ongoing, due tomorrow (6 PM) Midterm Exam 1 Grading is still underway... v This week s discussion session plan: Brief quiz on (all!) relational languages Chance to ask Q s about published HW solutions v Today s lecture plan: More about SQL...! Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2

2 Example Data in MySQL Sailors Reserves Boats Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 3 Find sid s of sailors who ve reserved a red or a green boat v If we replace OR by AND in this first version, what do we get? v UNION: Can be used to compute the union of any two union-compatible sets of tuples (which are themselves the result of SQL queries). v Also available: EXCEPT (What would we get if we replaced UNION by EXCEPT?) [Note: MySQL vs. RelaX and why?] Sailors(sid, sname, rating, age) Reserves(sid, bid, day) Boats(bid, bname, color) SELECT DISTINCT S.sid, Boats B, Reserves R AND (B.color= red OR B.color= green ) (SELECT S.sid, Boats B, Reserves R AND B.color= red ) UNION (SELECT S.sid, Boats B, Reserves R AND B.color= green ) Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 4

3 SQL vs. TRC Find sid s of sailors who ve reserved a red or a green boat Sailors(sid, sname, rating, age) SELECT S.sid, Boats B, Reserves R AND (B.color= red OR B.color= green ) Reserves(sid, bid, day) Boats(bid, bname, color) { t(sid) s Sailors (t.sid = s.sid r Reserves (r.sid = s.sid b Boats (b.bid = r.bid (b.color = red b.color = green )))) } Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 5 v Find sid s of sailors who ve reserved a red and a green boat INTERSECT: Can be used to compute the intersection of any two union-compatible sets of tuples. v Included in the SQL/92 standard, but not in all systems (incl. MySQL). v Contrast symmetry of the UNION and INTERSECT queries with how much the other versions differ. Sailors(sid, sname, rating, age) Reserves(sid, bid, day) Boats(bid, bname, color) SELECT S.sid, Boats B1, Reserves R1, Boats B2, Reserves R2 WHERE S.sid=R1.sid AND R1.bid=B1.bid AND S.sid=R2.sid AND R2.bid=B2.bid AND (B1.color= red AND B2.color= green Key field! SELECT S.sid, Boats B, Reserves R AND B.color= red INTERSECT SELECT S.sid, Boats B, Reserves R AND B.color= green Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 6

4 Nested Queries Find names of sailors who ve reserved boat #103: SELECT S.sname WHERE S.sid IN (SELECT R.sid FROM Reserves R WHERE R.bid=103) v A very powerful feature of SQL: a WHERE clause can itself contain an SQL query! (Actually, so can FROM and HAVING clauses!!) v To find sailors who ve not reserved #103, use NOT IN. v To understand semantics (including cardinality) of nested queries, think nested loops evaluation: For each Sailors tuple, check qualification by computing subquery. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 7 Nested Queries with Correlation Find names of sailors who ve reserved boat #103: SELECT S.sname WHERE EXISTS (SELECT * FROM Reserves R WHERE R.bid=103 AND S.sid=R.sid) v EXISTS is another set comparison operator, like IN. v Illustrates why, in general, subquery must be recomputed for each Sailors tuple (conceptually). NOTE: Recall that there was a join way to express this query, too. Relational query optimizers will try to unnest queries into joins when possible to avoid nested loop query evaluation plans. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 8

5 More on Set-Comparison Operators v We ve already seen IN and EXISTS.. Can also use NOT IN and NOT EXISTS. v Also available: op ANY, op ALL (for ops: >, <, =,,, ) v Find sailors whose rating is greater than that of some sailor called Horatio: SELECT * WHERE S.rating > ANY (SELECT S2.rating 2 WHERE S2.sname= Horatio ) Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 9 Rewriting INTERSECT Queries Using IN Find sid s of sailors who ve reserved both a red and a green boat: SELECT S.sid, Boats B, Reserves R AND B.color= red AND S.sid IN (SELECT S2.sid 2, Boats B2, Reserves R2 WHERE S2.sid=R2.sid AND R2.bid=B2.bid AND B2.color= green ) v Similarly, EXCEPT queries can be re-written using NOT IN. v This is what you ll need to do when using MySQL (but you can play with RelaX for the other set ops). Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 10

6 Division, SQL Style Find sailors who ve reserved all boats. (1) SELECT S.sname WHERE NOT EXISTS ((SELECT B.bid (This Sailor s unreserved Boat ids..!.) FROM Boats B) EXCEPT (SELECT R.bid FROM Reserves R WHERE R.sid=S.sid)) Sailors S such that... the set of all Boat ids... minus... this Sailor s reserved Boat ids... is empty! Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 11 Division in SQL (cont.) Find sailors who ve reserved all boats. v Let s do it the hard(er) way, i.e.., without EXCEPT: SELECT S.sname WHERE NOT EXISTS ((SELECT B.bid FROM Boats B) EXCEPT (SELECT R.bid FROM Reserves R WHERE R.sid=S.sid)) (2) SELECT S.sname WHERE NOT EXISTS (SELECT B.bid FROM Boats B Sailors S such that... WHERE NOT EXISTS (SELECT R.bid FROM Reserves R there is no boat B without... WHERE R.bid=B.bid AND R.sid=S.sid)) a Reserves tuple showing S reserved B Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 12 (1)

7 Aggregate Operators v Significant extension of relational algebra. COUNT (*) COUNT ( [DISTINCT] A) SUM ( [DISTINCT] A) AVG ( [DISTINCT] A) MAX (A) MIN (A) SELECT COUNT (*) SELECT AVG (S.age) WHERE S.rating=10 single column SELECT S.sname WHERE S.rating= (SELECT MAX(S2.rating) 2) SELECT COUNT (DISTINCT S.rating) WHERE S.sname= Bob SELECT AVG( DISTINCT S.age) WHERE S.rating=10 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 13 Find name and age of the oldest sailor(s) v The first query is illegal! (We ll look into the reason a bit later, when we discuss GROUP BY.) v The third query is equivalent to the second one, and allowed in the SQL/92 standard, but not supported in all systems. SELECT S.sname, MAX (S.age) SELECT S.sname, S.age WHERE S.age = (SELECT MAX (age) FROM Sailors) SELECT S.sname, S.age WHERE (SELECT MAX (S2.age) 2) = S.age Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 14

8 Motivation for Grouping v So far, we ve applied aggregate operators to all (qualifying) tuples. Sometimes, we want to apply them to each of several groups of tuples. v Consider: Find the age of the youngest sailor for each rating 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 (J): For i = 1, 2,..., 10: SELECT MIN (S.age) WHERE S.rating = i Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 15 Queries With GROUP BY and HAVING SELECT [DISTINCT] target-list FROM relation-list WHERE qualification GROUP BY grouping-list HAVING group-qualification v The target-list contains (i) attribute names and (ii) terms with aggregate operations (e.g., MIN (S.age)). The attribute list (i) must be a subset of grouping-list. Intuitively, each answer tuple corresponds to a group, and these attributes must have a single value per group. (A group is a set of tuples that have the same value for all attributes in grouping-list.) Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 16

9 Conceptual Evaluation v The cross-product of relation-list is computed, tuples that fail the qualification are discarded, `unnecessary fields are deleted, and the remaining tuples are partitioned into groups by the value of attributes in grouping-list. v A group-qualification (HAVING) is then applied to eliminate some groups. Expressions in groupqualification must also have a single value per group! In effect, an attribute in group-qualification that is not an argument of an aggregate op must appear in grouping-list. (Note: SQL does not consider primary key semantics here.) v One answer tuple is generated per qualifying group. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 17 Find age of the youngest sailor with age 18 for each rating with at least 2 such sailors. SELECT S.rating, MIN (S.age) AS minage WHERE S.age >= 18 GROUP BY S.rating HAVING COUNT (*) >= 2 Answer relation: rating minage Sailors instance: sid sname rating age 22 dustin brutus lubber andy rusty horatio zorba horatio art bob frodo Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 18

10 To Be Continued... J Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 19