sql slides(2)

8/8/2019 SQL Slides(2)

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SQL Query Slides

Sharif University Of TechnologyDatabase Systems

CE 384

Prepared By:Babak Bagheri [email protected]


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R etrieval Queries in SQL

Basic form of the SQL SELECT statement is called a mapping or aSELECT-FROM-WHERE block

SELECT < attribute list>FROM < table list>WHERE < condition>

< attribute list> is a list of attribute names whose values are toberetrieved by the query < table list> is a list of the relation names required to processthequery < condition> is a conditional (Boolean) expression that identifiesthe tuples to be retrieved by the query


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R elational Database schema


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Populated D atabase:


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Simple QueriesQ uery 0: Retrieve the birthdate and address of the employee whosename is 'John B. Smith'.

Q uery 1: Retrieve the name and address of all employees who work for the 'Research' department.

Q0: SELECT BDATE, ADDRESSFROM EMPLOYEE

WHERE FNAME='John' AND MINIT='B AND LNAME='Smith

Q1: SELECT FNAME, LNAME, ADDRESSFROM EMPLOYEE, DEPARTMENTWHERE DNAME='Research' AND

DNUMBER=DNO


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Some Queries Cont.

Q2: SELECT PNUMBER, DNUM, LNAME, BDATE, ADDRESSFROM PROJECT, DEPARTMENT, EMPLOYEEWHERE DNUM=DNUMBER AND MGRSSN=SSN AND

PLOCATION='Stafford'

Q uery 3: For each employee, retrieve the employee's name, and the nameof his or her immediate supervisor.

Q3: SELECT E.FNAME, E.LNAME, S.FNAME, S.LNAMEFROM EMPLOYEE E SWHERE E.SUPERSSN=S.SSN

Q uery 2: For every project located in 'Stafford', list the project number, thecontrolling department number, and the department manager's last name,address, and birthdate.


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Some Queries Cont.

Q4: (SELECT PNAMEFROM PROJECT, DEPARTMENT, EMPLOYEEWHERE DNUM=DNUMBER AND MGRSSN=SSN AND

LNAME='Smith')UNION (SELECT PNAMEFROM PROJECT, WORKS_ON, EMPLOYEEWHERE PNUMBER=PNO AND ESSN=SSN AND

LNAME='Smith')

Q uery 4: Make a list of all project numbers for projects that involve anemployee whose last name is 'Smith' as a worker or as a manager of the department that controls the project.


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Some Queries Cont.

Q5: SELECT E.FNAME, E.LNAMEFROM EMPLOYEE AS EWHERE E.SSN IN (SELECT ESSN

FROM DEPENDENTWHERE ESSN=E.SSN AND

E.FNAME=DEPENDENT_NAME)

Q uery 5: Retrieve the name of each employee who has a dependent

with the same first name as the employee.

Q5A: SELECT E.FNAME, E.LNAMEFROM EMPLOYEE E, DEPENDENT DWHERE E.SSN=D.ESSN AND

E.FNAME=D.DEPENDENT_NAME

The comparison operator IN compares a value v with a set (or multi-set)of values V, and evaluates to TRUE if v is one of the elements in V


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Some Queries Cont. EXISTS

Q5B: SELECT FNAME, LNAMEFROM EMPLOYEEWHERE EXISTS (SELECT *

FROM DEPENDENTWHERE SSN=ESSN AND

FNAME=DEPENDENT_NAME)

EXISTS is used to check whether the result of a correlated nested queryis empty (contains no tuples) or not


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Some Queries Cont.explicit ( enumerated) set of v alues

Q uery 6: Retrieve the social security numbers of all employees who workon project number 1, 2, or 3.

Q6: SELECT DISTINCT ESSN

FROM WORKS_ONWHERE PNO IN (1, 2, 3)

It is also possible to use an explicit (enumerated) set of values in theWHERE-clause rather than a nested query


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Some Queries Cont.

Q uery 7: Retrieve the name of each employee who works on all the projectscontrolled by department number 5.

Q7: SELECT FNAME, LNAMEFROM EMPLOYEEWHERE ( (SELECT PNO

FROM WORKS_ONWHERE SSN=ESSN)

CONTAINS(SELECT PNUMBERFROM PROJECTWHERE DNUM=5) )

The CONTAINS operator compares two sets of valu es , and returns TRUEif one set contains all values in the other set (reminiscent of the divi s ionoperation of algebra).


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Some Queries Cont. Null Value

Q uery 8: Retrieve the names of all employees who do not havesupervisors.

Q8: SELECT FNAME, LNAMEFROM EMPLOYEEWHERE SUPERSSN IS NULL

S Q L uses IS or IS NOT to compare NULLs because it considers each NULLvalue distinct from other NULL

Note: If a join condition is specified, tuples with NULL values for the joinattributes are not included in the result


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Some Queries Cont. JOIN

Can be written as:

QTA: SELECT E.FNAME, E.LNAME, S.FNAME, S.LNAMEFROM (EMPLOYEE E LEFT OUTER JOIN EMPLOYEESON E.SUPERSSN=S.SSN)

QT: SELECT E.FNAME, E.LNAME, S.FNAME, S.LNAMEFROM EMPLOYEE E SWHERE E.SUPERSSN=S.SSN


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Some Queries Cont. JOIN

Can be written as:

Q9A: SELECT FNAME, LNAME, ADDRESSFROM (EMPLOYEE JOIN DEPARTMENTON DNUMBER=DNO)WHERE DNAME='Research

Q9: SELECT FNAME, LNAME, ADDRESSFROM EMPLOYEE, DEPARTMENTWHERE DNAME='Research' AND DNUMBER=DNO

Or as:Q9B: SELECT FNAME, LNAME, ADDRESS

FROM (EMPLOYEE NATURAL JOINDEPARTMENT AS DEPT(DNAME, DNO, MSSN, MSDATE)

WHERE DNAME='Research


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Joined R elations Featurein SQL2

Q uery 2: For every project located in 'Stafford', list the project number, thecontrolling department number, and the department manager's last name,address, and birthdate.

Q2 B: SELECT PNUMBER, DNUM,LNAME, BDATE, ADDRESSFROM (PROJECT JOIN

DEPARTMENT ONDNUM=DNUMBER) JOIN

EMPLOYEE ONMGRSSN=SSN) )

WHERE PLOCATION='Stafford


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AGGR EGA TE FUNCTIONS

Q uery 10: Find the maximum salary, the minimum salary, and

the average salary among all employees.

Q10: SELECT MAX(SALARY), MIN(SALARY), AVG (SALARY)FROM EMPLOYEE

Include COUNT , SUM , MAX, MIN, and AVG

Q uery 11: Find the maximum salary, the minimum salary, and the average

salary among employees who work for the 'Research' department.Q11: SELECT MAX(SALARY), MIN(SALARY), AVG(SALARY)

FROM EMPLOYEE, DEPARTMENTWHERE DNO=DNUMBER AND

DNAME='Research'


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Group by

Q uery 12: For each department, retrieve the department number, thenumber of employees in the department, and their average salary.Q12: SELECT DNO, COUNT (*), AVG (SALARY)

FROM EMPLOYEEGROUP BY DNO

S Q L has a GROUP BY -clause for specifying the grouping attributes, whichm ust al s o app e ar in the SELECT-clau se

Q uery 13: For each project, retrieve the project number, project

name, and the number of employees who work on that project.Q13: SELECT PNUMBER, PNAME, COUNT (*)

FROM PROJECT, WORKS_ONWHERE PNUMBER=PNOGROUP BY PNUMBER, PNAME


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Group by cont. Having

Q uery 14: For each project on w h ic h m or e th an t wo e m ploy ees work ,retrieve the project number, project name, and the number of employeeswho work on that project.

Q14: SELECT PNUMBER, PNAME, COUNT (*)FROM PROJECT, WORKS_ON

WHERE PNUMBER=PNOGROUP BY PNUMBER, PNAMEHAVING COUNT (*) > 2

lThe HAVING-clause is used for specifying a selection condition on groups(rather than on individual tuples)


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Summary of SQL QueriesA query in SQL can consist of up to six clauses, butonly

the first two, SELECT and F R OM, are mandatory. Theclauses are specified in the following order:

SELECT < attribute list>FROM < table list>[ WHERE < condition>]

[G ROU P BY ][ HAVIN G ][ ORDER BY < attribute list>]


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Summary of SQL Queries

(cont.)The SELECT-clause lists the attributes or functions tobe retrievedThe F R OM-clause specifies all relations (or aliases)needed in the query but not those needed in nested

queriesThe WHE R E-clause specifies the conditions forselection and join of tuples from the relationsspecified in the F R OM-clauseGR OUP BY specifies grouping attributesHA VIN G specifies a condition for selection of groups

OR DER BY specifies an order for displaying the resultof a queryA query is evaluated by first applying the WHE R E-clause, thenGR OUP BY and H A VIN G , and finally the SELECT-clause


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More complex Select SQL Server

SELECT [ ALL | DISTINCT ][ TOP n [ PERCENT ] [ WITH TIES ] ]< select_list >< select_list > ::={ *| { t abl e_ na m e | vi e w _na m e | t abl e_ alia s }.*| { colu m n _na m e | ex pr ess ion | IDENTITYCOL |ROWGUIDCOL }[ [ AS ] colu m n _alia s ]| colu m n _alia s = ex pr ess ion} [ ,... n ]

SELECT se l e c t_ li st [ INTO ne w _t abl e ]FROM t abl e_s ourc e[ WHERE se arc h_ condi t ion ][ GROUP BY group _by _ex pr ess ion ][ HAVING se arc h_ condi t ion ][ ORDER BY ord e r _ex pr ess ion [ ASC |DESC ] ]Select Clause:

From Clause:[ FROM { < table_source > } [ ,... n ] ]< table_source > ::=t abl e_ na m e [ [ AS ] t abl e_ alia s ] [ WITH ( < table_hint > [ ,... n ]) ]| vi e w _na m e [ [ AS ] t abl e_ alia s ]

| row set_ func t ion [ [ AS ] t abl e_ alia s ]| OPENXML| d e riv e d _t abl e [ AS ] t abl e_ alia s [ ( colu m n _alia s [ ,... n ] ) ]| < joined_table >< joined_table > ::=< table_source > < join_type > < table_source > ON | < table_source > CROSS JOIN < table_source >| < joined_table >

< join_type > ::=[ INNER | { { LEFT | RIGHT | FULL } [ OUTER ] } ][ < join_hint > ]JOINArguments< table_source >


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More complex Select SQL Server

Cont.Where Clause:

[ WHERE < search_condition > | ]

< old_outer_join > ::=colu m n _na m e { * = | = * } colu m n _na m e

Group by clause:[ GROUP BY [ ALL ] group _by _ex pr ess ion [,... n ][ WITH { CUBE | ROLLUP } ]

]

Having:

[ HAVING < search_condition > ]

Order By Clause:

[ ORDER BY { order_by_expression [ ASC |DESC ] } [ ,... n ] ]

Compute Clause:[ COMPUTE{ { AVG | COUNT | MAX | MIN | STDEV | STDEVP| VAR | VARP | SUM }( ex pr ess ion ) } [ ,... n ][ BY ex pr ess ion [ ,... n ] ]]


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ComputeRow aggregate

function Result

AVG Average of the values in the numeric expression

COUNT Number of selected rows

MAX Highest value in the expression

MIN Lowest value in the expression

STDEV Statistical standard deviation for all values in the expression

STDEVP |Statistical standard deviation for the population for all values in the expression

SUM Total of the values in the numeric expression

VAR Statistical variance for all values in the expression

VARP Statistical variance for the population for all values in the expression

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