01-09-2007noea/it fen - databases/sql1 more about sql nested select other joins views index

01-09-2007 NOEA/IT FEN - Databases/SQL 1

More about SQL

Nested SELECT

Other Joins

Views

Index


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 <>).


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.


SQL2 - DML(Q7): Existential quantifier - EXISTS:

SELECT FNAME, LNAME

FROM EMPLOYEE

WHERE EXISTS (SELECT *

FROM DEPENDENT

WHERE SSN = ESSN)

AND

EXISTS (SELECT *

FROM DEPARTMENTWHERE

SSN = MGRSSN)


SQL2 - DML(Q6): NOT EXISTS:

SELECT FNAME, LNAME

FROM EMPLOYEE

WHERE NOT EXISTS (SELECT *

FROM DEPENDENT

WHERE SSN =ESSN)


SQL2 - DML For 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 ---


FROM ---

WHERE NOT EXISTS ----


• Let x be 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 ---


FROM ---

WHERE NOT EXISTS ----

A Side: Predicate Logic


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))


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.


SQL2 - DML

SELECT DISTINCT ESSN

FROM WORKS_ON

WHERE PNO IN (1,2,3)

SELECT FNAME, LNAME

FROM EMPLOYEE

WHERE SUPERSSN IS NULL


SQL2 - DML

SELECTE.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 in FROM part.


SQL2 - DML

Alternative notations for join:

SELECT FNAME, LNAME, ADDRESSFROM(EMPLOYEE JOIN DEPARTMENT ON

DNO=DNUMBER)WHERE DNAME = ’Research’

Provides a more clear syntax and opens for more specialised joins.


SQL2 - DML

Natural join (not MS SQL Server):(Q1B):SELECT FNAME, LNAME, ADDRESSFROM (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).


SQL2 - DML

Outer join:


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.


FROM (EMPLOYEE E LEFT OUTER JOIN EMPLOYEE S

ON E.SUPERSSN = S.SSN)

Also RIGTH OUTER JOIN and FULL OUTER JOIN.


SQL2 - DML


What about employees with no

supervisor?


Here they are!


SQL2 - DML


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


SQL2 - DML

Aggregate Functions:

– COUNT– SUM– MAX– MIN– AVG


SQL2 - DMLEx.: ”Number of Employees in the research department”

SELECT COUNT(*)

FROM EMPLOYEE, DEPARTMENT

WHERE DNO = DNUMBER

AND DNAME = ’Research’


SQL2 - DML(Q24)


Result of Q24


SQL2 - DML(Q26)


Result of Q26, 1


Result of Q26, 2


SQL2 - DML


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.


SQL - VIEWSCREATE VIEW WORKS_ON1

AS SELECT FNAME, LNAME, PNAME, HOURSFROM EMPLOYEE, PROJECT, WORKS_ONWHERE SSN=ESSN AND PNO=PNUMBER;

Using this view, the query:

SELECT FNAME, LNAME, PNAMEFROM EMPLOYEE, PROJECT, WORKS_ONWHERE PNAME = 'ProductX'

AND SSN = ESSN AND PNO = PNUMBER;May written as

SELECT FNAME, LNAME, PNAMEFROM WORKS_ON1WHERE PNAME = 'ProductX';

And hence saving the join


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’


SQL - VIEWSWhich update of the base tables should be executed?This?

Or this?


SQL - VIEWS

Views and update:

Updatableviews

Gray zone

Not updatable views


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.


Index• Index is often considered part of the DDL of SQL

– index is about internal physical storage access, and– as such NOT a part of the SQL2 standard– many SQL versions include index anyway

• Previous versions of SQL includes a CREATE INDEX statement– Is used for speeding up queries, for instance:

CREATE INDEX LNAME_INDEX ON EMPLOYEE(LNAME);

– Index is in some older SQL implementations also used to simulate

primary keys:

CREATE UNIQUE INDEX SSN_INDEX ON EMPLOYEE(SSN);


Index

– Often it is possible to define one (and only one) clustering index:

CREATE INDEX DNO_INDEX

ONEMPLOYEE(DNO)

CLUSTER;

– The effect is that employees in the same department are stored physically close

– Hereby queries concerning employees in the same department are speeded up.

Why is that?


Databaser (SQL) og programudviklingsværktøjer

DBMS/

SQL 3GL

Embedded SQL

OOPL 4GL (skærm- og

rapportgeneratorer, eget sprog)

Visuelle programmerings-

værktøjer (Visual Studio, Delphi, J-

Builder…)

ODBC/JDBC giver standardiseret tilgang til RDBMS


Databaseprogrammering fra 3GLs og OOPLs

• SQL og typiske programmeringssprog som Java, C, Cobol etc. er ikke ”kompatible”:

• Impedans-mismatch:– Programmeringssprogets datatyper er ikke afstemt med

databasens domæner– SQL er mængde- (tabel-) orienteret:

• alle resultater er en tabel med et antal tupler.• 3GLs og OOPLs er tuple- (objekt-)orienteret, dvs.• der er behov for en datastruktur i

programmeringssproget


Embedded SQL

Databaseaccess vha. SQL fra 3GL/OOPL (værtssprog):

Er baseret Cursor-begrebet:• En cursor kan opfattes som en pointer, der udpeger aktiv række i

tabellen, som SQL-sætningen resulterer i• En cursor er nødvendig, når resultatet af en SQL-sætning er mere

end een tuple (værtssproget er record- (objekt-) orienteret)• Er leverandørafhængig• Visse moderne værktøjer (fx Visual Studio.NET) kan

tilsyneladende returnere en mængde af forekomster (dataset eller ResultSet, men reelt er der tale om en indpakket cursor

• ODBC (Open Data Base Connectivity): Resultset svarer til en cursor.

• I OOPLs kaldes en cursor ofte en iterator (Enumerator i C#).

01-09-2007noea/it fen - databases/sql1 more about sql nested select other joins views index

Documents

noeait fen databasessql7

noeait fen databasessql16

noeait fen databasessql17

research slide

essn slide

null slide

avg slide

mgrssn slide