CMPT 258 Database Systems SQL Queries (Chapter 5)

2 Agenda Introduction Basic SQL Query Union, Intersection and Except Nested Queries Aggregate Operations

Examples Reserves Sailors Boats 3

Basic SQL Query relation-list A list of relation names (possibly with a range-variable after each name). target-list A list of attributes of relations in relation-list qualification Comparisons (Attr op const or Attr1 op Attr2, where op is one of ) combined using AND and OR. SELECT target-list FROM relation-list WHERE qualification

The query uses the optional keyword AS to introduce a range variable. 5

Basic SQL Query DISTINCT is an optional keyword indicating that the answer should not contain duplicates. ▫Default is that duplicates are not eliminated! SELECT [DISTINCT] target-list FROM relation-list WHERE qualification

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Conceptual Evaluation Strategy Semantics of an SQL query defined in terms of the following conceptual evaluation strategy: ▫ Compute the cross-product of relation-list. ▫ Discard resulting tuples if they fail qualifications. ▫ Delete attributes that are not in target-list. ▫ If DISTINCT is specified, eliminate duplicate rows. SELECT [DISTINCT] target-list FROM relation-list WHERE qualification

Reserves Sailors Boats

A Note on Range Variables Really needed only if the same relation appears twice in the FROM clause. The previous query can also be written as: SELECT S.sname FROM Sailors S, Reserves R WHERE S.sid=R.sid AND R.bid=103 SELECT sname FROM Sailors, Reserves WHERE Sailors.sid=Reserves.sid AND bid=103 It is good style, however, to use range variables always! OR

13 Query Examples Find the sids of sailors who have reserved a red boat

14 Query Examples Find the sids of sailors who have reserved a red boat Find the name of sailors who have reserved a red boat SELECT R.sid FROM Reserves R, Boats B WHERE R.bid=B.bid AND B.color = ‘red’

15 Query Examples Find the sids of sailors who have reserved a red boat Find the name of sailors who have reserved a red boat SELECT R.sid FROM Reserves R, Boats B WHERE R.bid=B.bid AND B.color = ‘red’ SELECT S.sname FROM Sailors S, Reserves R, Boats B WHERE S.sid=R.sid AND R.bid = B.bid AND B.color = ‘red’

Find the color of the boats reserved by Lubber 16

Find the color of the boats reserved by Lubber 17 SELECT B.color FROM Sailors S, Reserves R, Boats B WHERE S.sid=R.sid AND R.bid = B.bid AND S.name = ‘Lubber’

Find the names of sailors who’ve reserved at least one boat

SELECT DISTINCT S.name FROM Sailors S, Reserves R WHERE S.sid=R.sid

20 Find the names of sailors who have sailed two different boats on the same day.

21 Find the names of sailors who have sailed two different boats on the same day. SELECT S.sname FROM Sailors S, Reserves R1, Reserves R2 WHERE S.sid= R1.sid and R1.sid=R2.sid AND R1.day = R2.day AND R1.bid <> R2.bid

Expressions and Strings Find ages of sailors whose names begin and end with B and contain at least three characters. LIKE is used for string matching. ‘_’ stands for any one character and ‘%’ stands for 0 or more arbitrary characters (including blanks). ‘_AB%’ Comparison operators (=,, etc.) can be used for string comparison

Expressions and Strings Find ages of sailors whose names begin and end with B and contain at least three characters. SELECT S.age FROM Sailors S WHERE S.sname LIKE ‘B_%B’

Query FROM More than One Table Using JOINs A join combines the rows of two tables, based on a rule called a join condition; this compares values from the rows of both tables to determine which rows should be joined. Basic types of join: ▫inner join ▫outer join  LEFT OUTER JOIN  RIGHT OUTER JOIN  FULL OUTER JOIN 24

Inner Join select a, b from A, B where a=b 26

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Reserves Sailors Boats

Union Intersect Except 31

Find sid’s of sailors who’ve reserved a red or a green boat

UNION : Can be used to compute the union of any two union- compatible sets of tuples (which are themselves the result of SQL queries). SELECT R.sid FROM Boats B, Reserves R WHERE R.bid=B.bid AND (B.color=‘red’ OR B.color=‘green’)

Find sid’s of sailors who’ve reserved a red or a green boat UNION : Can be used to compute the union of any two union- compatible sets of tuples (which are themselves the result of SQL queries). If we replace OR by AND in the first version, what do we get? SELECT R.sid FROM Boats B, Reserves R WHERE R.bid=B.bid AND (B.color=‘red’ OR B.color=‘green’) SELECT R.sid FROM Boats B, Reserves R WHERE R.bid=B.bid AND B.color=‘red’ UNION SELECT R.sid FROM Boats B, Reserves R WHERE R.bid=B.bid AND B.color=‘green’

35 Find the sids of sailors who have a rating of 10 or reserved boat 104

36 Find the sids of sailors who have a rating of 10 or reserved boat 104 The default for UNION queries is that duplicates are eliminated. To retain duplicates, use UNION ALL SELECT S.sid FROM Sailors S WHERE S.rating = 10 UNION SELECT R.sid FROM Reserves R WHERE R.bid=104

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. Some systems don’t support it. SELECT R.sid FROM Boats B, Reserves R WHERE R.bid=B.bid AND B.color=‘red’ INTERSECT SELECT R.sid FROM Boats B, Reserves R WHERE R.bid=B.bid AND B.color=‘green’

38 Find the sids of all sailors who have reserved red boats but not green boats SELECT R.sid FROM Boats B, Reserves R WHERE R.bid=B.bid AND B.color=‘red’ EXCEPT SELECT R.sid FROM Boats B, Reserves R WHERE R.bid=B.bid AND B.color=‘green’

39 Find the names of sailors who’ve reserved a red and a green boat Is this query correct? Why SELECT S.sname FROM Sailors S, Reserves R, Boats B WHERE S.sid=R.sid AND R.bid=B.bid AND B.color=‘red’ INTERSECT SELECT S.sname FROM Sailors S, Boats B, Reserves R WHERE S.sid=R.sid AND R.bid=B.bid AND B.color=‘green’

40 Find the names of sailors who’ve reserved a red and a green boat Is this query correct? Why SELECT S.sname FROM Sailors S, Reserves R, Boats B WHERE S.sid=R.sid AND R.bid=B.bid AND B.color=‘red’ INTERSECT SELECT S.sname FROM Sailors S, Boats B, Reserves R WHERE S.sid=R.sid AND R.bid=B.bid AND B.color=‘green’ There are two sailors with the same name Horatio!

Readings Chapter 5