Tài liệu displaying data from Multiple tables docx

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 4Ć2... Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 4Ć4... Introduction to Oracle: SQL and PL/SQL Using P

Displaying Data from Multiple

Tables4

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 4Ć2

This lesson will cover how to obtain data from more than one table, using the many different methods available.

At the end of this lesson, you should be able to

D Write SELECT statements to access data from more than one table using equalityand non-equality joins

D View data that would not normally meet a join condition by using outer joins

D Join a table to itself

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 4Ć4

When data from more than one table in the database is required, a join condition is

used Rows in one table may be joined to rows in another table according to commonvalues existing in corresponding columns, that is to say primary and foreign keycolumns

There are two main types of join conditions:

For more information about set operators, attend

Advanced SQL and SQL*Plus course.

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 4Ć6

What Is a Cartesian Product?

When a join condition is invalid or omitted completely, the result is a Cartesian

Product, in which all combinations of rows will be displayed All rows in the first

table are joined to all rows in the second table

And Why Should You Care?

A Cartesian product tends to generate a large number of rows, and its result is rarelyuseful You should always include a valid join condition in a WHERE clause, unlessyou have a specific need to combine all rows from all tables

SQL> SELECT name, last_name

300 rows selected

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 4Ć8

Simple Join Query

To display data from two or more related tables, write a simple join condition in theWHERE clause

Syntax

SELECT table.column, table.column

where: table.column denotes the table and column from which data is

retrieved

table1.column1 = is the condition that joins (or relates)

table2.column2 the tables together

to uniquely identify each row

For more information, see

Oracle7 Server SQL Language Reference Manual, “SELECT.”

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 4Ć10

Server

ÉÉÉÉ

Simple Join Query continued

Equijoin

In order to determine the name of an employee’s department, you compare the value

in the DEPT_ID column in the S_EMP table with the ID values in the S_DEPT table

The relationship between the S_EMP and S_DEPT tables is an equijoin, that is

values in the DEPT_ID column on both tables must be equal Frequently, these

columns are primary and foreign key complements

4 WHERE s_emp.dept_id = s_dept.id;

LAST_NAME DEPT_ID NAME

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 4Ć12

Simple Join Query continued

Qualifying Ambiguous Column Names

You need to qualify the names of the columns in the WHERE clause with the tablename to avoid ambiguity Without the table prefixes, the ID column could be fromeither the S_DEPT or the S_EMP table It is necessary to add the table prefix toexecute your query

If there are no names that are the same between the two tables, then there is no need

to qualify the columns However, you will gain improved performance by using thetable prefix

Example

Display the department number, region number, and region name for all departments

SQL> SELECT s_dept.id ”Department ID”,

5 WHERE s_dept.region_id = s_region.id;

Department ID Region ID Region Name

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 4Ć14

Server

ÉÉ ÉÉ

Simple Join Query continued

Additional Search Conditions

In addition to the join, you may have additional criteria for your WHERE clause.Since the join is required to obtain the matches, you need to add your additionalconditions by using the AND operator Table aliases help to keep SQL code smaller,therefore using less memory

Example

Display employee Menchu’s last name, department number, and department name

SQL> SELECT s_emp.last_name, s_emp.dept_id,

4 WHERE s_emp.dept_id = s_dept.id

LAST_NAME DEPT_ID NAME

LAST_NAME NAME COMMISSION_PCT

- -

-Magee North America 10

Giljum South America 12.5

Sedeghi Africa / Middle East 10

Nguyen Asia 15

Dumas Europe 17.5

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 4Ć16

Using Table Aliases

Qualifying column names with table names can be very time consuming, particularly

if table names are lengthy Use table aliases instead Like column aliases, table

aliases are a method of giving the table another name for the purpose of the SELECT

statement Once you use the table alias, you must continue to qualify every columnreference with the table alias

Example

Display the customer name, region number, and region name for all customers.Provide column aliases, and use a table alias to shorten the table references

SQL> SELECT c.name ”Customer Name”,

D Table aliases should be meaningful

D The table alias is only valid for the current SELECT statement

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 4Ć18

The relationship between the EMP and SALGRADE tables is a non-equijoin, in that

no column in EMP corresponds directly to a column in SALGRADE The

relationship is obtained using an operator other than equal (=).

Example

Create a non-equijoin to evaluate an employee’s salary grade The salary must be

between any pair of the low and high salary ranges.

SQL> SELECT e.ename, e.job, e.sal, s.grade

3 WHERE e.sal BETWEEN s.losal AND s.hisal;

ENAME JOB SAL GRADE

- - - -

SMITH CLERK 800.00 1

ADAMS CLERK 1,100.00 1

JAMES CLERK 950.00 1

WARD SALESMAN 1,250.00 2

MARTIN SALESMAN 1,250.00 2

MILLER CLERK 1,300.00 2

ALLEN SALESMAN 1,600.00 3

TURNER SALESMAN 1,500.00 3

JONES MANAGER 2,975.00 4

BLAKE MANAGER 2,850.00 4

CLARK MANAGER 2,450.00 4

SCOTT ANALYST 3,000.00 4

FORD ANALYST 3,000.00 4

KING PRESIDENT 5,000.00 5

14 rows selected

Other operators such as <= and >= could be used, however BETWEEN is the

simplest Remember to specify the low value first and the high value last when using BETWEEN Table aliases have been specified for performance reasons, not because

of possible ambiguity

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 4Ć20

Returning Records with No Direct Match

If a row does not satisfy a join condition, then the row will not appear in the query

result For example, in the equijoin condition of S_EMP and S_CUSTOMER, SweetRock Sports does not appear because there is no sales representative for that

customer

Outer Join

The missing row(s) can be returned if an outer join operator is used in the join

condition The operator is a plus sign enclosed in parentheses (+), and is placed on

the “side” of the join that is deficient in information The operator has the effect of

creating one or more NULL rows, to which one or more rows from the non-deficienttable can be joined

Syntax

SELECT table.column, table.column

WHERE table1.column = table2.column(+);

or

SELECT table.column, table.column

WHERE table1.column(+) = table2.column;

where: table1.column = is the condition that joins (or relates) the tables

table2.column together

(+) is the outer join symbol; it can be placed on

either side of the WHERE clause condition, butnot on both sides Place the outer join symbolfollowing the name of the table without thematching rows

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 4Ć22

Server

ÉÉ É

Returning Records with No Direct Match continued

Example

Display the sales representative name and employee number and the customer namefor all customers Include the customer name even if the customer has not beenassigned a sales representative

SQL> SELECT e.last_name, e.id, c.name

3 WHERE e.id (+) = c.sales_rep_id

4 ORDER BY e.id;

LAST_NAME ID NAME

–––––––– ––– ––––––––––––––––––––––––––––––––––––

Magee 11 Womansport

Magee 11 Beisbol Si!

Magee 11 Ojibway Retail

Magee 11 Big John’s Sports Emporium

Giljum 12 Unisports

Giljum 12 Futbol Sonora

Sedeghi 13 Hamada Sport

Nguyen 14 Simms Atheletics

Nguyen 14 Delhi Sports

Dumas 15 Kam’s Sporting Goods

Dumas 15 Sportique

Dumas 15 Muench Sports

Dumas 15 Sporta Russia

Dumas 15 Kuhn’s Sports

Sweet Rock Sports

15 rows selected.

Outer Join Restrictions

D The outer join operator can only appear on one side of the expression—the side

that has information missing It returns those rows from one table which have nodirect match in the other table

D A condition involving an outer join may not use the IN operator or be linked toanother condition by the OR operator

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 4Ć24

Server

ÉÉ É

Joining a Table to Itself

You can join a table to itself by using table aliases to simulate as if the table were twoseparate tables This allows rows in a table to be joined to rows in the same table

Display the names of employees and their respective managers

SQL> SELECT worker.last_name||’ works for ’||

4 WHERE worker.manager_id = manager.id;

Ngao works for Velasquez

Nagayama works for Velasquez

Quick-To-See works for Velasquez

Ropeburn works for Velasquez

Urguhart works for Ngao

Menchu works for Ngao

Biri works for Ngao

Catchpole works for Ngao

Havel works for Ngao

Magee works for Nagayama

Giljum works for Nagayama

Sedeghi works for Nagayama

Nguyen works for Nagayama

Dumas works for Nagayama

24 rows selected

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 4Ć26

There are multiple ways to join tables The common thread, though, is that you want

to link them through a condition in the WHERE clause The method you choose will

be based on the required result and the data structures you are using

Syntax

SELECT table.column, table.column

where: table.column denotes a table and column from which data is

retrieved

table1.column1 = is the condition that joins (or relates)

table2.column2 the tables together

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 4Ć28

Practice Overview

This practice is intended to give you practical experience extracting data from morethan one table You will be required to join and restrict rows in the WHERE clause

Practice Contents

D Joining tables using an equijoin

D Performing outer and self joins

D Adding additional conditions

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 4Ć30

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 4Ć32

-Magee Sales North America

Giljum Sales South America

Sedeghi Sales Africa / Middle East

Nguyen Sales Asia

Dumas Sales Europe

3. Display the employee name and department name for Smith

-Grand Prix Bicycle Tires 30421 15

Pro Curling Bar 40422 30

Prostar 10 Pound Weight 41010 20

Prostar 100 Pound Weight 41100 35

Major League Baseball 50169 40

Griffey Glove 50417 27

Cabrera Bat 50530 50

7 rows selected

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 4Ć34

If you have time, complete the following exercises

6. Display the customer number, customer name, and order number of all customers and their orders Display the customer number and name, even if they have not placed an order

Customer ID Customer Name Order ID

-

201 Unisports 97

202 Simms Atheletics 98

203 Delhi Sports 99

204 Womansport 100

204 Womansport 111

205 Kam’s Sporting Goods 101

206 Sportique 102

207 Sweet Rock Sports

208 Muench Sports 103

208 Muench Sports 104

209 Beisbol Si! 105

210 Futbol Sonora 106

210 Futbol Sonora 112

211 Kuhn’s Sports 107

212 Hamada Sport 108

213 Big John’s Sports Emporium 109

214 Ojibway Retail 110

215 Sporta Russia

18 rows selected

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 4Ć36

8. Modify the solution to exercise 7 to also display Velasquez, who has no manager

26 rows selected.

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 4Ć38