1 / 36

More about SQL

More about SQL. More about SELECT, Nested selects GROUP BY, HAVING, ORDER BY Other joins Aggregate functions Views. SQL2 - DML. (Q16): Sub queries (nested SELECTs) SELECT E.FNAME, E.LNAME FROM EMPLOYEE E WHERE E.SSN IN (SELECT ESSN FROM DEPENDENT WHERE ESSN = E.SSN

amal
Download Presentation

More about SQL

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. More about SQL More about SELECT, Nested selects GROUP BY, HAVING, ORDER BY Other joins Aggregate functions Views FEN 2014-04-27

  2. SQL2 - DML (Q16): Sub queries (nested SELECTs) SELECT E.FNAME, E.LNAME FROM EMPLOYEE E WHERE E.SSN IN (SELECT ESSN FROM DEPENDENT WHERE ESSN = E.SSN AND E.FNAME = DEPENDENT_NAME AND SEX = E.SEX) Also ANY (SOME) and ALL in combination with comparison operators (>, >=, <, >= and <>). FEN 2014-04-27

  3. SQL2 - DML (Q16): Sub queries (nested SELECTs) SELECT E.FNAME, E.LNAME FROM EMPLOYEE E WHERE E.SSN IN (SELECT ESSN FROM DEPENDENT WHERE ESSN = E.SSN AND E.FNAME = DEPENDENT_NAME AND SEX = E.SEX) For each row in outer table (E), the inner SELECT is executed. If E.SSN is contained in the result of the inner SELECT, then E is included in the result table for the outer SELECT. FEN 2014-04-27

  4. SQL2 - DML (Q7): Existential quantifier - EXISTS: SELECT FNAME, LNAME FROM EMPLOYEE WHERE EXISTS(SELECT * FROM DEPENDENT WHERE SSN = ESSN) AND EXISTS (SELECT * FROM DEPARTMENT WHERE SSN = MGRSSN) FEN 2014-04-27

  5. SQL2 - DML (Q6): NOT EXISTS: SELECT FNAME, LNAME FROM EMPLOYEE WHERE NOT EXISTS (SELECT * FROM DEPENDENT WHERE SSN =ESSN) FEN 2014-04-27

  6. SQL2 - DMLFor All i SQL • Although SQL is supposed to be an implementation of first order predicate logic, it does not support the universal qualifier (FORALL), only the existential quantifier (EXISTS) is supported. • A well known (?) result from predicate logic can be used in a workaround: • Retrieving all elements satisfying some predicate is equivalent to retrieving elements that are not in the set of elements that do not satisfy the predicate: SELECT * FROM --- WHERE NOT EXISTS (SELECT * FROM --- WHERE NOT EXISTS ---- FEN 2014-04-27

  7. A Side: Predicate Logic • Let x an arbitrary element in some set and p a predicate stating some condition on x: • De Morgan’s Law: (x: p(x))  x: p(x) Apply to p(x): (x: p(x))  x: (p(x)) Reduce the right hand side: x: p(x)  (x: p(x)) “it is not true that there exists x, so p(x) is not true” – that is: “p is true for all x” SELECT * FROM --- WHERE NOT EXISTS (SELECT * FROM --- WHERE NOT EXISTS ---- FEN 2014-04-27

  8. SQL2 - DML (Q3B): ”Retrieve the name of each employee who works on all projects controlled by department number 5” SELECT LNAME, FNAME FROM EMPLOYEE WHERE NOT EXISTS (SELECT * FROM WORKS_ON B WHERE (B.PNO IN (SELECT PNUMBER FROM PROJECT WHERE DNUM = 5)) AND NOT EXISTS (SELECT * FROM WORKS_ON C WHERE C.ESSN = SSN AND C.PNO=B.PNO)) FEN 2014-04-27

  9. SQL2 - DML SELECT DISTINCT ESSN FROM WORKS_ON WHERE PNO IN (1,2,3) SELECT FNAME, LNAME FROM EMPLOYEE WHERE SUPERSSN IS NULL FEN 2014-04-27

  10. SQL2 - DML SELECT E.LNAME AS EMP_NAME, S.LNAME AS SUPER_NAME FROM EMPLOYEE AS E, EMPLOYEE AS S WHERE E.SUPERSSN = S.SSN New coulomb names in the resulting table. AS may be omitted. FEN 2014-04-27

  11. SQL2 - DML Alternative notations for join: SELECT FNAME, LNAME, ADDRESS FROM (EMPLOYEE JOIN DEPARTMENT ON DNO=DNUMBER) WHERE DNAME = ’Research’ Provides a more clear syntax and opens for more specialised joins. FEN 2014-04-27

  12. SQL2 - DML Natural join (not MS SQL Server): (Q1B): SELECT FNAME, LNAME, ADDRESS FROM (EMPLOYEE NATURAL JOIN (DEPARTMENT AS DEPT (DNAME,DNO,MSSN,MSDATE))) WHERE DNAME = ’Research’ DEPARTMENT.DNUMBER must be rename to DNO in order to match EMPLOYEE.DNO. Natural join is over two attributes with the same name (EMPLOYEE.DNO = DEPT.DNO). FEN 2014-04-27

  13. SQL2 - DML Outer join: SELECT E.LNAME AS EMP_NAME, S.LNAME AS SUPER_NAME FROM EMPLOYEE AS E, EMPLOYEE AS S WHERE E.SUPERSSN = S.SSN Retrieves only employees who have a supervisor. Left Outer Join retrieves all employees and inserts NULL in the S-attributes for employees with no supervisor. SELECT E.LNAME AS EMP_NAME, S.LNAME AS SUPER_NAME FROM (EMPLOYEE E LEFT OUTER JOIN EMPLOYEE S ON E.SUPERSSN = S.SSN) Also RIGTH OUTER JOIN and FULL OUTER JOIN. FEN 2014-04-27

  14. SQL2 - DML FEN 2014-04-27

  15. What about employees with no supervisor? FEN 2014-04-27

  16. Here they are! FEN 2014-04-27

  17. SQL2 - DML FEN 2014-04-27

  18. SQL2 - DML FEN 2014-04-27

  19. SQL2 - DML Also: • CROSS JOIN (Cartesian Product) • UNION JOIN • SQL2 provides many different ways of expressing the same join: • This can be view as an advantage: • More simple expressions • Or as an disadvantage: • More complicated language FEN 2014-04-27

  20. SQL2 – DML: SELECT Queries: SELECT <attribute-list> FROM <tables> [WHERE <condition>] [GROUP BY <attribute-list>] [HAVING <condition>] [ORDER BY <attribute-list>] [...]: WHERE, GROUP BY, HAVING and ORDER BY may be omitted. FEN 2014-04-27

  21. SQL2 - DML Aggregate Functions: • COUNT • SUM • MAX • MIN • AVG How are NULLs treated? AVG(-) == SUM(-)/COUNT(-) ??? FEN 2014-04-27

  22. SQL2 - DML Ex.: ”Number of Employees in the research department” SELECT COUNT(*) FROM EMPLOYEE, DEPARTMENT WHERE DNO = DNUMBER AND DNAME = ’Research’ FEN 2014-04-27

  23. SQL2 - DML(Q24) Try this one with at least one employee with a null value in salary. Compare the result with the query: Select DNO, COUNT(*), SUM(SALARY)/COUNT(*) FEN 2014-04-27

  24. Result of Q24 FEN 2014-04-27

  25. SQL2 - DML(Q26) FEN 2014-04-27

  26. Result of Q26, 1 FEN 2014-04-27

  27. Result of Q26, 2 FEN 2014-04-27

  28. SQL2 - DML FEN 2014-04-27

  29. Break for Exercises • Elmasri: 5.5a FEN 2014-04-27

  30. SQL - VIEWS • A view is virtual table which is created from one or more existing base tables. • Views may be used in a layered architecture to provide different view of the database to different users. • May also be used to increase efficiency of frequent queries, for instance to avoid JOINs. FEN 2014-04-27

  31. SQL - VIEWS CREATE VIEW WORKS_ON1 AS SELECT FNAME, LNAME, PNAME, HOURS FROM EMPLOYEE, PROJECT, WORKS_ON WHERE SSN=ESSN AND PNO=PNUMBER; Using this view, the query: SELECT FNAME, LNAME, PNAME FROM EMPLOYEE, PROJECT, WORKS_ON WHERE PNAME = 'ProductX' AND SSN = ESSN AND PNO = PNUMBER; May written as SELECT FNAME, LNAME, PNAME FROM WORKS_ON1 WHERE PNAME = 'ProductX'; And hence saving the join FEN 2014-04-27

  32. SQL - VIEWS • Updating through views is problematic: • FX: Transfer John Smith from the project 'ProductX' to the project 'ProductY’ UPDATE WORKS_ON1 SET PNAME = ’ProductY’ WHERE LNAME = ’Smith’ AND FNAME = ’John’ AND PNAME = ’ProductX’ FEN 2014-04-27

  33. SQL - VIEWS Which update of the base tables should be executed? This? Or this? FEN 2014-04-27

  34. Not updatable vies Gray zone Updatable views SQL - VIEWS Views and update: FEN 2014-04-27

  35. SQL - VIEWS Generally: • Views defined over one base table can be updated, if the primary key (ore some candidate key) is included in the view. • Views defined by joining more base tables are generally not updatable. • Some joined view are in principle updatable: all primary keys from the base tables must be included in the view. • Views defined using aggregate or grouping functions are not updatable. • SQL2 standard establishes that joined view are not updatable. FEN 2014-04-27

  36. Exercises • From session03 • Investigate how MS SQL Server implements some of the SQL constructs treated to day • Elmasri 5.7 • Elmasri 5.8, 5.9 • Try out some of the queries in solutions • Extra: FlereSQLOpgaver.pdf FEN 2014-04-27

More Related