Database Programming

Sections 3 – Oracle Joins

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Obtaining Data from Multiple Tables: Using Joins

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Types of Joins Oracle Proprietary

Joins (8i and prior): Cartesian Product Equijoin Non-equijoin Outer Join Self join

SQL: 1999 Compliant Joins: Cross joins Natural joins Using clause Full or Two sided

outer joins Arbitrary join

conditions for outer

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Joining Tables Using Oracle Syntax Use a join to query data from more than

one table. SELECT table1.column, table2.column

FROM table1, table2WHERE table1.column1 = table2.column2; Write the join condition in the WHERE clause. Prefix the column name with the table name

when the same column name appears in more than one table.

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Cartesian Products A Cartesian product is formed when:

A join condition is omitted A join condition is invalid All rows in the first table are joined to all

rows in the second table To avoid a Cartesian product, always

include a valid join condition in a WHERE clause.

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Generating a Cartesian Product SELECT last_name,department_name

FROM employees,departments Employees table has 20 rows Departments table has 8 rows Cartesian product: 20x8=160 rows

Run the query

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What is an Equijoin? Employees

Departments

Employee_ID Department_ID

200 10201 20202 20124 50141 50

Department_ID Department_name

10 Administration

20 Marketing

20 Marketing

50 Shipping

50 Shipping

60 IT

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Retrieving Records with Equijoins SELECT employees.employee_id, employees.last_name,

employees.department_id, departments.department_id, departments.location_idFROM employees,departmentsWHERE employees.department_id = departments.department_id;

19 rows retrieved in following example.

EMPLOYEE_ID LAST_NAME DEPARTMENT_ID DEPARTMENT_ID LOCATION_ID

100 King 90 90 1700

101 Kochhar 90 90 1700

102 De Haan 90 90 1700

103 Hunold 60 60 1400

104 Ernst 60 60 1400

107 Lorentz 60 60 1400

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Using the AND Operator SELECT last_name, employees.department_id, department_name

FROM employees,departmentsWHERE employees.department_id = departments.department_id AND last_name = 'Matos';

LAST_NAMEDEPARTMENT_ID

DEPARTMENT_ID DEPARTMENT_NAME

Ernst 60 10 Administration

Lorentz 60 20 Marketing

Davies 50 50 Shipping

Matos 50 60 IT

Vargas 50 80 Sales

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Ambiguous Column Names SELECT last_name, department_id,

location_idFROM employees, departments;

Run the above Note that both tables contain

department_id column and thus the query is ambiguous

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Qualifying Ambiguous Column Names Use table prefixes to qualify column

names that are in multiple tables. Improve performance by using table

prefixes. Distinguish columns that have

identifical names but reside in different tables by using column aliases.

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Generating a Cartesian Product SELECT last_name, d.department_id,

location_idFROM employees e, departments d; Employees table has 20 rows Departments table has 8 rows Cartesian product: 20x8=160 rows

Run the query

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Using Tables Aliases Simplify queries by using tables aliases. Improve performance by using table

prefixes. SELECT e.employee_id, e.last_name,

e.department_id, d.department_id, d.location_idFROM employees e,departments dWHERE e.department_id = d.department_id;

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Joining More than Two Tables To join n tables together, you need a minimum of n-1 join

conditions. For example, to join three tables, a minimum of two joins is required.

LAST_NAMEDEPARTMENT_ID

DEPARTMENT_ID DEPARTMENT_NAME Location_id City

Ernst 60 10 Administration 1400 Southlake

Lorentz 60 20 Marketing 1700 Seattle

Davies 50 50 Shipping 1800 Toronto

Matos 50 60 IT 2500 Oxford

Vargas 50 80 Sales

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Three Table Join Example SELECT e.last_name, d.department_name, l.city

FROM employees e,departments d, locations lWHERE e.department_id = d.department_id AND d.location_id = l.location_id;

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Non-EquijoinsLAST_NAME

SALARY

Ernst 6000

Lorentz 4200

Davies 3100

Matos 2600

Vargas 2500

Ziotkey 10500

Salary in the EMPLOYEES table must be between lowest salary and highest salary in the JOB_GRADES table.

EMPLOYEES table JOB_GRADES table

GPA LOWEST_SAL HIGHEST_SAL

A 1000 2999

B 3000 5999

C 6000 9999

D 10000 14999

E 15000 24999

F 25000 40000

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Retrieving Records with Non-Equijoins Run the following query: SELECT e.last_name, e.salary,

j.grade_levelFROM employees e, job_grades jWHERE e.salary BETWEEN j.lowest_sal AND j.highest_sal;

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Retrieving Records with Non-Equijoins SELECT e.last_name, e.salary

FROM employees e, job_grades jWHERE e.salary > j.lowest_sal AND e.salary < 5800;

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Self Joins Manager_id in the Worker table is equal to

Employee_id in the Manager table. Employees (Worker) Employees (Manager)

Employee_id LAST_NAMEManager_ID

Employee_id LAST_NAME

100 King 100 King

101 Kochhar 100 101 Kochhar

102 De Haan 100 102 De Haan

103 Hunold 102 103 Hunold

104 Ernst 103 104 Ernst

107 Lorentz 103 107 Lorentz

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Joining a table to itself example SELECT worker.last_name || ' works for '

|| manager.last_nameFROM employees worker, employees managerWHERE worker.manager_id = manager.employee_id;

Run the above to see results.

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Outer Joins There are no employees in department 190. Departments table Employees table

DEPARTMENT_NAMEDEPARTMENT_ID

DEPARTMENT_ID LAST_NAME

Administration 10 90 King

Marketing 20 90 Kochhar

Shipping 50 90 De Haan

IT 60 60 Hunold

Sales 80 60 Ernst

Contracting 190 50 Rajs

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Outer Join example SELECT d.department_name,

e.last_nameFROM departments d, employees eWHERE d.department_id = e.department_id AND d.department_id = 190;

Run this and you will see there are no employees in the 190 department.

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Outer Join Syntax You use an outer join to also see rows

that do not meet the join condition. The Outer join operator is the plus

sign (+). SELECT table1.column, table2.column

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

SELECT table1.column, table2.columnFROM table1, table2,WHERE table1.column = table2.column (+);

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Using Outer Joins - examples SELECT e.last_name, e.department_id,

d.department_nameFROM employees e, departments dWHERE e.department_id(+) = d.department_id;