Chapter 8 Database Redesign We only discuss complex sub-queries

Database Processing: Fundamentals, Design, and Implementation, 9/e by David M. Kroenke 2 Referring to itself  Find the student names used by more than one student First try Select name From S Group By Name Having count(*) > 1 Second Try Select s.name Form S, S as S2 Where s.name = s2.name and s.sid <> s2.SID

Database Processing: Fundamentals, Design, and Implementation, 9/e by David M. Kroenke 3 Referring to itself (2)  Find the pair of SIDs using the same name Select S.Name, S.SID, S2.SID Form S, S as S2 Where s.name = s2.name and s.sid < s2.SID Note: you cannot use subquery or “Group By” to do this one

Database Processing: Fundamentals, Design, and Implementation, 9/e by David M. Kroenke 4 EXISTS and NOT EXISTS  EXISTS and NOT EXISTS are specialized forms of subqueries An EXISTS condition is true if any row in the subquery meets the specified conditions A NOT EXISTS condition is true only if all rows in the subquery do not meet the specified condition  Double use of NOT EXISTS can be used to find rows that have some specified condition to every row of a table

Database Processing: Fundamentals, Design, and Implementation, 9/e by David M. Kroenke 5 Example: EXISTS  List the names of students who take at least one class SELECT Name FROM S WHERE EXISTS (SELECT * FROM E WHERE SID =StudentNumber );  What is the difference? SELECT Name FROM S WHERE SID in (SELECT StudentNumber FROM E );

Database Processing: Fundamentals, Design, and Implementation, 9/e by David M. Kroenke 6 Example: NOT EXISTS  List the names of students who take no class SELECT Name FROM S WHERE NOT EXISTS (SELECT * FROM E WHERE SID =StudentNumber );  What is the difference? SELECT Name FROM S WHERE SID NOT in (SELECT StudentNumber FROM E );

Database Processing: Fundamentals, Design, and Implementation, 9/e by David M. Kroenke 7 Example: NOT EXISTS -- ALL  List the names of students who take all classes SELECT Name FROM S WHERE SID in (SELECT StudentNumber FROM E group by student number Having count(*) = (select count(*) from C) );

Database Processing: Fundamentals, Design, and Implementation, 9/e by David M. Kroenke 8 Example: NOT EXISTS – ALL (2)  List the names of students who take all classes  List the names of students for whom there does not exist a class the students are not taking SELECT Name FROM S WHERE Not Exists (SELECT * FROM C where not exists (select * from E where c.name = className and Sid = studentnumber );

Database Processing: Fundamentals, Design, and Implementation, 9/e by David M. Kroenke 9 Example: NOT EXISTS – ALL (3)  List the names of students who take all classes taking by PAKERs SELECT Name FROM S WHERE Not Exists (SELECT * FROM E as E2 where studentnumber in (select SID from S where S.name = ‘Paker’) and not exists (select * from E where E2.Classname = E.className and Sid = studentnumber );