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SQL: Queries, Constraints, Triggers Chapter 5 – Part 1

SQL: Queries, Constraints, Triggers Chapter 5 – Part 1. Agenda. Introduction Basic SQL Query Union, Intersection and Except Nested Queries Aggregate Operations Null Values Complex Integrity Constraints in SQL Triggers and Active Databases Design Active Database. About the examples.

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SQL: Queries, Constraints, Triggers Chapter 5 – Part 1

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  1. SQL: Queries, Constraints, Triggers Chapter 5 – Part 1

  2. Agenda • Introduction • Basic SQL Query • Union, Intersection and Except • Nested Queries • Aggregate Operations • Null Values • Complex Integrity Constraints in SQL • Triggers and Active Databases • Design Active Database

  3. About the examples

  4. Example Instances R1 S1 S2 • We will use these instances of the Sailors and Reserves relations in our examples. • If the key for the Reserves relation contained only the attributes sid and bid, how would the semantics differ?

  5. 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, OR and NOT. • DISTINCT is an optional keyword indicating that the answer should not contain duplicates. • Default is that duplicates are not eliminated! SELECT [DISTINCT] target-list FROMrelation-list WHERE qualification

  6. 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. • There are three basic types of join: • inner join, created with the INNER JOIN keywords • outer join, which comes in three varieties: • LEFT OUTER JOIN • RIGHT OUTER JOIN • FULL OUTER JOIN • cross join, created with the CROSS JOIN keywords

  7. Inner Join • Select a, b • From A, B • Where A.a=B.b

  8. Select a, b • From A, B

  9. 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. • This strategy is probably the least efficient way to compute a query! • An optimizer will find more efficient strategies to compute the same answers.

  10. Example of Conceptual Evaluation Select S.sname From Sailors as S inner join Reserves as R on S.sid = R.sid Where R.bid = 103

  11. 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 bid=103 It is good style, however, to use range variables always! OR SELECT sname FROM Sailors, Reserves WHERE Sailors.sid=Reserves.sid AND bid=103

  12. Query Examples • Find the sids of sailors who have reserved a red boat (Q16) • Find the name of sailors who have reserved a red boat (Q2) SELECT B.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’

  13. Find the color of the boats reserved by Lubber (Q3) SELECT B.color FROM Sailors S, Reserves R, Boats B WHERE S.sid=R.sid AND R.bid = B.bid AND S.name = ‘Lubber’

  14. Find sailors who’ve reserved at least one boat • Would adding DISTINCT to this query make a difference? • What is the effect of replacing S.sid by S.sname in the SELECT clause? • Would adding DISTINCT to this variant of the query make a difference? SELECT S.sid FROM Sailors S, Reserves R WHERE S.sid=R.sid

  15. Expressions and Strings • Find triples (of ages of sailors and two fields defined by expressions) for sailors whose names begin and end with B and contain at least three characters. • Illustrates use of arithmetic expressions and string pattern matching: • ASand = are two ways to name fields in result. • LIKE is used for string matching. `_’ stands for any one character and `%’ stands for 0 or more arbitrary characters. • Comparison operators (=, <, >, etc.) can be used for string comparison SELECT S.age, age1=S.age-5, 2*S.age AS age2 FROM Sailors S WHERE S.sname LIKE ‘B_%B’

  16. Expressions and Strings • Compute the increments for the ratings of persons who have sailed two different boats on the same day. • Each item in a qualification can be as general as expression1=expression2 SELECT S.sname, S.rating+1 as rating FROM Sailors S, Reserves R1, Reserves R2 WHERE S.sid=R1.sid AND S.sid = R2.sid AND R1.day = R2.day AND R1.bid <> R2.bid SELECT S.sname AS name1, S2.sname AS name2 FROM Sailors S1, Sailors S2 WHERE 2*S1.rating = S2.rating - 1

  17. (Q18) Find the ages of sailors whose name begins and ends with B and has at least three characters

  18. Union • Intersect • except

  19. 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 ORby ANDin the first version, what do we get? • Also available: EXCEPT(What do we get if we replace UNIONby EXCEPT?) SELECT S.sid FROM Sailors S, Boats B, Reserves R WHERE S.sid=R.sid AND R.bid=B.bid AND (B.color=‘red’ OR B.color=‘green’) SELECT S.sid FROM Sailors S, Boats B, Reserves R WHERE S.sid=R.sid AND R.bid=B.bid AND B.color=‘red’ UNION SELECT S.sid FROM Sailors S, Boats B, Reserves R WHERE S.sid=R.sid AND R.bid=B.bid AND B.color=‘green’

  20. 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. • Contrast symmetry of the UNION and INTERSECT queries with how much the other versions differ. SELECT S.sid FROM Sailors S, 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’) SELECT S.sid FROM Sailors S, Boats B, Reserves R WHERE S.sid=R.sid AND R.bid=B.bid AND B.color=‘red’ INTERSECT SELECT S.sid FROM Sailors S, Boats B, Reserves R WHERE S.sid=R.sid AND R.bid=B.bid AND B.color=‘green’

  21. 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 S2.sname FROM Sailors S2, Boats B2, Reserves R2 WHERE S2.sid=R2.sid AND R2.bid=B2.bid AND B2.color=‘green’ There are two sailors with the same name Horatio!

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

  23. Find the sids of sailors who have a rating of 10 or reserved boat 104 • In contrast to the default that duplicates are not eliminated unless DISTINCT is specified in the basic query form, the default for UNION queries is that duplicates are eliminated! • To retain duplicates, use UNION ALL, INTERSECT ALL and EXCEPT ALL SELECT S.sid FROM Sailors S WHERE S.rating = 10 UNION SELECT R.sid FROM Reserves R WHERE R.bid=104

  24. Questions?

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