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SQL-examples.ppt
1.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-1 Summary of SQL Queries A query in SQL can consist of up to six clauses, but only the first two, SELECT and FROM, are mandatory. The clauses are specified in the following order: SELECT <attribute list> FROM <table list> [WHERE <condition>] [GROUP BY <grouping attribute(s)>] [HAVING <group condition>] [ORDER BY <attribute list>]
2.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-2 Summary of SQL Queries (cont.) The SELECT-clause lists the attributes or functions to be retrieved The FROM-clause specifies all relations (or aliases) needed in the query but not those needed in nested queries The WHERE-clause specifies the conditions for selection and join of tuples from the relations specified in the FROM-clause GROUP BY specifies grouping attributes HAVING specifies a condition for selection of groups ORDER BY specifies an order for displaying the result of a query A query is evaluated by first applying the WHERE-clause, then GROUP BY and HAVING, and finally the SELECT-clause
3.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-3 THE HAVING-CLAUSE Query: For each project on which more than two employees work , retrieve the project number, project name, and the number of employees who work on that project. SELECT PNUMBER, PNAME, COUNT (*) FROM PROJECT, WORKS_ON WHERE PNUMBER=PNO GROUP BY PNUMBER, PNAME HAVING COUNT (*) > 2
4.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-4
5.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-5 SQL> select * from employee; NAME SSN SALARY DNO SUPERSSN ----- ---------- ---------- ---------- ---------- johny 12345 40000 5 33344 frank 33344 30000 5 88866 james 88866 40000 4 77777 borgs 77777 30000 5 88866 (Zhang 55555 35000 4 12345 TTTT 66666 10000 1 33344) Q: count total # of employees whose salaries exceed $35000, but only for dept where more than 1 employee works.
6.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-6 SQL> select dname,count(*) 2 from dept,employee 3 where dnumber=dno and salary >35000 4 group by dname 5 having count(*) > 1; no rows selected
7.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-7 SQL> select dname,count(*) 2 from dept,employee 3 where dnumber=dno and salary >35000 and 4 dno in (select dno 5 from employee 6 group by dno 7 having count(*) > 1) 8 group by dname; DNAME COUNT(*) ----- ---------- resea 1
8.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-8 SQL> select dname,count(*),sum(salary) 2 from dept,employee 3 where dnumber=dno and 4 dno in (select dno 5 from employee 6 group by dno 7 having count(*) > 1) 8 group by dname 9 having sum(salary) > 35000; DNAME COUNT(*) SUM(SALARY) ----- ---------- ----------------- resea 3 100000
9.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-9 SQL> select * from employee1; FNAME LNAME SSN SUPERSSN ----- ----- ---------- ---------- johny smith 12345 33344 frank whong 33344 88866 james borgs 88866 SQL> select * from depedent; FNAME LNAME SSN AGE ----- ----- ---------- ---------- johny smith 12345 4 frank whong 33344 6 johny smith 12345 14 james borgs 88866 16 james borgs 88866 18
10.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-10 NESTING OF QUERIES A complete SELECT query, called a nested query , can be specified within the WHERE-clause of another query, called the outer query Many of the previous queries can be specified in an alternative form using nesting Query : Retrieve the name and address of all employees who work for the 'Research' department. Q: SELECT FNAME, LNAME, ADDRESS FROM EMPLOYEE WHERE DNO IN (SELECT DNUMBER FROM DEPARTMENT WHERE DNAME='Research' )
11.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-11 NESTING OF QUERIES The nested query selects the number of the 'Research' department The outer query select an EMPLOYEE tuple if its DNO value is in the result of either nested query 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 In general, we can have several levels of nested queries A reference to an unqualified attribute refers to the relation declared in the innermost nested query In this example, the nested query is not correlated with the outer query
12.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-12 CORRELATED NESTED QUERIES If a condition in the WHERE-clause of a nested query references an attribute of a relation declared in the outer query , the two queries are said to be correlated The result of a correlated nested query is different for each tuple (or combination of tuples) of the relation(s) the outer query Query: Retrieve the name of each employee who has a dependent with the same first name as the employee. Q: SELECT E.FNAME, E.LNAME FROM EMPLOYEE AS E WHERE E.SSN IN (SELECT ESSN FROM DEPENDENT WHERE ESSN=E.SSN AND E.FNAME=DEPENDENT_NAME)
13.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-13 SQL> select FNAME, LNAME 2 from employee1 E 3 where (select count(*) 4 from depedent 5 where ssn = E.ssn ) >= 2; where (select count(*) * ERROR at line 3: ORA-00936: missing expression SQL> select FNAME, LNAME 2 from employee1 E 3 where 2 <= (select count(*) 4 from depedent 5 where ssn = E.ssn ); FNAME LNAME ----- ----- johny smith james borgs
14.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-14 ORDER BY The ORDER BY clause is used to sort the tuples in a query result based on the values of some attribute(s) Query: Retrieve a list of employees and the projects each works in, ordered by the employee's department, and within each department ordered alphabetically by employee last name. Q: SELECT DNAME, LNAME, FNAME, PNAME FROM DEPARTMENT, EMPLOYEE, WORKS_ON, PROJECT WHERE DNUMBER=DNO AND SSN=ESSN AND PNO=PNUMBER ORDER BY DNAME, LNAME
15.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-15 ORDER BY (cont.) The default order is in ascending order of values We can specify the keyword DESC if we want a descending order; the keyword ASC can be used to explicitly specify ascending order, even though it is the default
16.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-16 Specifying Updates in SQL There are three SQL commands to modify the database; INSERT, DELETE, and UPDATE
17.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-17 INSERT In its simplest form, it is used to add one or more tuples to a relation Attribute values should be listed in the same order as the attributes were specified in the CREATE TABLE command
18.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-18 INSERT (cont.) Example: U1: INSERT INTO EMPLOYEE VALUES ('Richard','K','Marini', '653298653', '30-DEC-52', '98 Oak Forest,Katy,TX', 'M', 37000,'987654321', 4 ) An alternate form of INSERT specifies explicitly the attribute names that correspond to the values in the new tuple Attributes with NULL values can be left out Example: Insert a tuple for a new EMPLOYEE for whom we only know the FNAME, LNAME, and SSN attributes. U1A: INSERT INTO EMPLOYEE (FNAME, LNAME, SSN) VALUES ('Richard', 'Marini', '653298653')
19.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-19 INSERT (cont.) Important Note: Only the constraints specified in the DDL commands are automatically enforced by the DBMS when updates are applied to the database Another variation of INSERT allows insertion of multiple tuples resulting from a query into a relation
20.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-20 INSERT (cont.) – Example: Suppose we want to create a temporary table that has the name, number of employees, and total salaries for each department. A table DEPTS_INFO is created by U3A, and is loaded with the summary information retrieved from the database by the query in U3B. U3A: CREATE TABLE DEPTS_INFO (DEPT_NAME VARCHAR(10), NO_OF_EMPS INTEGER, TOTAL_SAL INTEGER); U3B: INSERT INTO DEPTS_INFO (DEPT_NAME, NO_OF_EMPS, TOTAL_SAL) SELECT DNAME, COUNT (*), SUM (SALARY) FROM DEPARTMENT, EMPLOYEE WHERE DNUMBER=DNO GROUP BY DNAME ;
21.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-21 INSERT (cont.) Note: The DEPTS_INFO table may not be up-to-date if we change the tuples in either the DEPARTMENT or the EMPLOYEE relations after issuing U3B. We have to create a view (see later) to keep such a table up to date.
22.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-22 DELETE Removes tuples from a relation Includes a WHERE-clause to select the tuples to be deleted Tuples are deleted from only one table at a time (unless CASCADE is specified on a referential integrity constraint) A missing WHERE-clause specifies that all tuples in the relation are to be deleted; the table then becomes an empty table The number of tuples deleted depends on the number of tuples in the relation that satisfy the WHERE-clause Referential integrity should be enforced
23.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-23 DELETE (cont.) Examples: U4A: DELETE FROM EMPLOYEE WHERE LNAME='Brown’ U4B: DELETE FROM EMPLOYEE WHERE SSN='123456789’ U4C: DELETE FROM EMPLOYEE WHERE DNO IN (SELECT DNUMBER FROM DEPARTMENT WHERE DNAME='Research') U4D: DELETE FROM EMPLOYEE
24.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-24 UPDATE Used to modify attribute values of one or more selected tuples A WHERE-clause selects the tuples to be modified An additional SET-clause specifies the attributes to be modified and their new values Each command modifies tuples in the same relation Referential integrity should be enforced
25.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-25 UPDATE (cont.) Example: Change the location and controlling department number of project number 10 to 'Bellaire' and 5, respectively. U5: UPDATE PROJECT SET PLOCATION = 'Bellaire', DNUM = 5 WHERE PNUMBER=10
26.
Elmasri and Navathe,
Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe Slide 8-26 UPDATE (cont.) Example: Give all employees in the 'Research' department a 10% raise in salary. U6: UPDATE EMPLOYEE SET SALARY = SALARY *1.1 WHERE DNO IN (SELECT DNUMBER FROM DEPARTMENT WHERE DNAME='Research') In this request, the modified SALARY value depends on the original SALARY value in each tuple The reference to the SALARY attribute on the right of = refers to the old SALARY value before modification The reference to the SALARY attribute on the left of = refers to the new SALARY value after modification
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