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Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke1 SQL: Queries, Programming, Triggers Chapter 5 Modified by Donghui Zhang.

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Presentation on theme: "Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke1 SQL: Queries, Programming, Triggers Chapter 5 Modified by Donghui Zhang."— Presentation transcript:

1 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke1 SQL: Queries, Programming, Triggers Chapter 5 Modified by Donghui Zhang

2 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke2 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 FROM relation-list WHERE qualification

3 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke3 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.

4 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke4 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 SELECT sname FROM Sailors, Reserves WHERE Sailors.sid=Reserves.sid AND bid=103 It is good style, however, to use range variables always! OR

5 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke5 Expressions and Strings  Illustrates use of arithmetic expressions and string pattern matching: 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.  AS and = 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. SELECT S.age, age1=S.age-5, 2*S.age AS age2 FROM Sailors S WHERE S.sname LIKE ‘B_%B’

6 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke6 Set Operations in SQL  UNION: s1 UNION s2, result rows either in s1 or s2.  INTERSECT: s1 INTERSECT s2, result rows in s1 and s2.  EXCEPT: s1 EXCEPT s2, result rows in s1 but not in s2. (Some system recognize ‘MINUS’ for EXECPT)  Also, IN, ANY, ALL, EXISTS, to be covered in ‘Nested Queries’.

7 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke7 Nested Queries  A very powerful feature of SQL: a WHERE clause can itself contain an SQL query! (Actually, so can FROM and HAVING clauses.)  To find sailors who’ve not reserved #103, use NOT IN.  To understand semantics of nested queries, think of a nested loops evaluation: For each Sailors tuple, check the qualification by computing the subquery. SELECT S.sname FROM Sailors S WHERE S.sid IN ( SELECT R.sid FROM Reserves R WHERE R.bid=103) Find names of sailors who’ve reserved boat #103:

8 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke8 Nested Queries with Correlation  EXISTS is another set comparison operator, like IN.  If UNIQUE is used, and * is replaced by R.bid, finds sailors with at most one reservation for boat #103. ( UNIQUE checks for duplicate tuples; * denotes all attributes. Why do we have to replace * by R.bid ?)  Illustrates why, in general, subquery must be re- computed for each Sailors tuple. SELECT S.sname FROM Sailors S WHERE EXISTS ( SELECT * FROM Reserves R WHERE R.bid=103 AND S.sid=R.sid) Find names of sailors who’ve reserved boat #103:

9 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke9 More on Set-Comparison Operators  We’ve already seen IN, EXISTS and UNIQUE. Can also use NOT IN, NOT EXISTS and NOT UNIQUE.  Also available: op ANY, op ALL, op IN  Find sailors whose rating is greater than that of some sailor called Horatio: SELECT * FROM Sailors S WHERE S.rating > ANY ( SELECT S2.rating FROM Sailors S2 WHERE S2.sname=‘Horatio’)

10 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke10 Division in SQL  Let’s do it the hard way, without EXCEPT : SELECT S.sname FROM Sailors S WHERE NOT EXISTS (( SELECT B.bid FROM Boats B) EXCEPT ( SELECT R.bid FROM Reserves R WHERE R.sid=S.sid)) SELECT S.sname FROM Sailors S WHERE 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)) Sailors S such that... there is no boat B without... a Reserves tuple showing S reserved B Find sailors who’ve reserved all boats. (1) (2)

11 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke11 Aggregate Operators  Significant extension of relational algebra. COUNT (*) COUNT ( [ DISTINCT ] A) SUM ( [ DISTINCT ] A) AVG ( [ DISTINCT ] A) MAX (A) MIN (A) SELECT AVG (S.age) FROM Sailors S WHERE S.rating=10 SELECT COUNT (*) FROM Sailors S SELECT AVG ( DISTINCT S.age) FROM Sailors S WHERE S.rating=10 SELECT S.sname FROM Sailors S WHERE S.rating= ( SELECT MAX (S2.rating) FROM Sailors S2) single column SELECT COUNT ( DISTINCT S.rating) FROM Sailors S WHERE S.sname=‘Bob’

12 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke12 Queries With GROUP BY and HAVING  The target-list contains (i) attribute names (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.) SELECT [DISTINCT] target-list FROM relation-list WHERE qualification GROUP BY grouping-list HAVING group-qualification

13 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke13 Conceptual Evaluation  The cross-product of relation-list is computed, tuples that fail qualification are discarded, ` unnecessary’ fields are deleted, and the remaining tuples are partitioned into groups by the value of attributes in grouping-list.  The group-qualification is then applied to eliminate some groups. Expressions in group-qualification must have a single value per group !  In effect, an attribute in group-qualification that is not an argument of an aggregate op also appears in grouping-list. (SQL does not exploit primary key semantics here!)  One answer tuple is generated per qualifying group.

14 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke14 Find the age of the youngest sailor with age 18, for each rating with at least 2 such sailors  Only S.rating and S.age are mentioned in the SELECT, GROUP BY or HAVING clauses; other attributes ` unnecessary ’.  2nd column of result is unnamed. (Use AS to name it.) SELECT S.rating, MIN (S.age) FROM Sailors S WHERE S.age >= 18 GROUP BY S.rating HAVING COUNT (*) >= 2 Answer relation

15 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke15 Find the age of the youngest sailor with age 18, for each rating with at least 2 such sailors SELECT T.rating, T.minimum FROM (SELECT S.rating, MIN(S.age) AS minimum, COUNT (*) AS ratingcount FROM Sailors S WHERE S.age > 18 GROUP BY S.rating) AS T WHERE T.ratingcount >= 2 SELECT S.rating, MIN (S.age) FROM Sailors S WHERE S.age >= 18 GROUP BY S.rating HAVING COUNT (*) >= 2

16 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke16 Null Values  Field values in a tuple 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.  Is rating>8 true or false when rating is equal to null ? What about AND, OR and NOT connectives?  We 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.

17 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke17 Integrity Constraints (Review)  An IC describes conditions that every legal instance of a relation must satisfy.  Inserts/deletes/updates that violate IC’s are disallowed.  Can be used to ensure application semantics (e.g., sid is a key), or prevent inconsistencies (e.g., sname has to be a string, age must be < 200)  Types of IC’s : Domain constraints, primary key constraints, foreign key constraints, general constraints.  Domain constraints : Field values must be of right type. Always enforced.

18 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke18 General Constraints  Useful when more general ICs than keys are involved.  Can use queries to express constraint.  Constraints can be named. CREATE TABLE Sailors ( sid INTEGER, sname CHAR(10), rating INTEGER, age REAL, PRIMARY KEY (sid), CHECK ( rating >= 1 AND rating <= 10 ) CREATE TABLE Reserves ( sname CHAR(10), bid INTEGER, day DATE, PRIMARY KEY (bid,day), CONSTRAINT noInterlakeRes CHECK (`Interlake’ <> ( SELECT B.bname FROM Boats B WHERE B.bid=bid)))

19 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke19 Constraints Over Multiple Relations CREATE TABLE Sailors ( sid INTEGER, sname CHAR(10), rating INTEGER, age REAL, PRIMARY KEY (sid), CHECK ( ( SELECT COUNT (S.sid) FROM Sailors S) + ( SELECT COUNT (B.bid) FROM Boats B) < 100 )  Awkward and wrong!  If Sailors is empty, the number of Boats tuples can be anything!  ASSERTION is the right solution; not associated with either table. CREATE ASSERTION smallClub CHECK ( ( SELECT COUNT (S.sid) FROM Sailors S) + ( SELECT COUNT (B.bid) FROM Boats B) < 100 ) Number of boats plus number of sailors is < 100

20 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke20 Triggers  Trigger: procedure that starts automatically if specified changes occur to the DBMS  Three parts:  Event (activates the trigger)  Condition (tests whether the triggers should run)  Action (what happens if the trigger runs)

21 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke21 Triggers: Example (SQL:1999) CREATE TRIGGER youngSailorUpdate AFTER INSERT ON SAILORS REFERENCING NEW TABLE NewSailors FOR EACH STATEMENT INSERT INTO YoungSailors(sid, name, age, rating) SELECT sid, name, age, rating FROM NewSailors N WHERE N.age <= 18

22 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke22 Exercises Sailors ( sid, sname, rating, age ) Boats ( bid, bname, color ) Reserve( sid, bid, date )

23 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke23 Find snames whose rating>10 and who reserved some red boat. Solution 1: 3-way join. Solution 2: use IN: … WHERE S.rating>10 AND S.sid IN Solution 3: use EXISTS … WHERE S.rating>10 AND EXISTS … Solution 4: use INTERSECT … WHERE S.sid IN ( … INTERSECT … )

24 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke24 Find snames whose rating>10 or who reserved some red boat. Simple modifications from the above four cases. In particular: INTERSECT  UNION Solution 5: union on snames.

25 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke25 Find sids with the highest rating. Solution 1: use MAX … S.rating = (SELECT MAX(S2.rating) …) Solution 2: use ALL … S.rating >= ALL ( SELECT S2.rating … ) Solution 3: use EXCEPT … EXCEPT ( … S.sid < ANY (SELECT S2.rating) … )

26 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke26 Find bids not reserved by sid=5. Solution 1: use EXCEPT Solution 2: use NOT EXISTS Find sids who reserved all boats. Find sids who reserved all red boats.

27 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke27 Find sids who made exactly one reservation. Solution 1: use NOT EXISTS not exists another reserve record with same sid. Solution 2: use UNIQUE … WHERE UNIQUE (… WHERE R.sid=R2.sid) Solution 3: use GROUPBY & HAVING

28 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke28 More examples on groupby. - For each sid who reserved, find # reservations. - For each sid who reserved at least 5 times, find # different boats she reserved. - For each sid who reserved, find # different colors she reserved. - For each bid which was reserved by at least two different sailors, find avg rating of reservers.

29 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke29 More examples on nested query as source. - Find the ratings for which the average age of sailors is minimum over all ratings. - Find the sailor sids who made more than average number of reservations per sailor. * choice 1: counting only those in Reserves. * choice 2: including those who did not reserve.

30 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke30 Exercise 5.2 Suppliers ( sid : integer, sname : string, address : string ) Parts ( pid : integer, pname : string, color : string ) Catalog ( sid : integer, pid : integer, cost : real )

31 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke31 4. Find the pnames of parts supplied by Acme Widget Suppliers and no one else. SELECT P.pname FROM Parts P, Catalog C, Suppliers S WHERE P.pid = C.pid AND C.sid = S.sid AND S.sname = `Acme Widget Suppliers' AND NOT EXISTS ( SELECT * FROM Catalog C1, Suppliers S1 WHERE P.pid = C1.pid AND C1.sid = S1.sid AND S1.sname <> `Acme Widget Suppliers' )

32 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke32 3. Find the snames of suppliers who supply every red part. SELECT S.sname FROM Suppiers S WHERE NOT EXISTS ( (SELECT P.pid FROM Parts P WHERE P.color = `red' ) EXCEPT (SELECT C.pid FROM Catalog C WHERE C.sid = S.sid) )

33 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke33 6. For each part, find the sname of the supplier who charges the most for that part. SELECT P.pid, S.sname FROM Parts P, Suppliers S, Catalog C WHERE C.pid = P.pid AND C.sid = S.sid AND C.cost = (SELECT MAX (C1.cost) FROM Catalog C1 WHERE C1.pid = P.pid)

34 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke34 7. Find the sids of suppliers who supply only red parts. SELECT DISTINCT C.sid FROM Catalog C WHERE NOT EXISTS ( SELECT * FROM Catalog C1, Parts P WHERE C1.pid = C.pid AND C1.pid = P.pid AND P.color <> `red' )

35 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke For every supplier that only supplies green parts, print the name of the supplier and the total number of parts that she supplies. SELECT S.sname, COUNT(*) as PartCount FROM Suppliers S, Parts P, Catalog C WHERE P.pid = C.pid AND C.sid = S.sid GROUP BY S.sname, S.sid HAVING EVERY (P.color='green')

36 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke36 Exercise 5.7 Emp ( eid : integer, ename : string, age : integer, salary : real ) Works ( eid : integer, did : integer, pct_time : integer ) Dept ( did : integer, budget : real, managerid : integer )

37 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke37 1. Define a table constraint on Emp that will ensure that every employee makes at least $10,000. CREATE TABLE Emp ( eid INTEGER, ename CHAR(10), age INTEGER, salary REAL, PRIMARY KEY (eid), CHECK ( salary >= ))

38 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke38 3. Define an assertion on Dept that will ensure that all managers have age > 30. Compare this assertion with the equivalent table constraint. Explain which is better. CREATE TABLE Dept ( did INTEGER, budget REAL, managerid INTEGER, PRIMARY KEY (did) ) CREATE ASSERTION managerAge CHECK ( (SELECT MIN(E.age) FROM Emp E, Dept D WHERE E.eid = D.managerid ) > 30 )

39 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke39 Exercise 5.10 Emp ( eid : integer, ename : string, age : integer, salary : real ) Works ( eid : integer, did : integer, pct_time : integer ) Dept ( did : integer, budget : real, managerid : integer )  An employee can work in more than one department; the pct_time field of the Works relation shows the percentage of time that a given employee works in a given department.  Write integrity constraints or triggers.

40 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke40 2. Every manager must also be an employee. CREATE ASSERTION ManagerIsEmployee CHECK ( ( SELECT COUNT (*) FROM Dept D WHERE D.managerid NOT IN (SELECT eid FROM Emp)) = 0)

41 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke41 3. The total percentage of all appointments for an employee must be under 100%. CREATE TABLE Works ( eid INTEGER, did INTEGER, pct time INTEGER, PRIMARY KEY (eid, did), CHECK ( (SELECT COUNT (W.eid) FROM Works W GROUP BY W.eid HAVING Sum(pct time) > 100) = 0))

42 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke42 5. Whenever an employee is given a raise, the manager’s salary must be increased to be at least as much. CREATE TRIGGER GiveRaise AFTER UPDATE ON Emp WHEN old.salary < new.salary FOR EACH ROW BEGIN UPDATE Emp M SET M.Salary = new.salary WHERE M.salary < new.salary AND M.eid IN (SELECT D.mangerid FROM Emp E, Works W, Dept D WHERE E.eid = new.eid AND E.eid = W.eid AND W.did = D.did); END


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