Answer Posted / tanu

UNION: Union Combines results of all select statements,
displaying duplicate rows only one time in the answerset.

Rules for usage:
1. All select clauses must have same number of expressions.
2. Corresponding expressions must have compatible domains.

First SELECT statement:
->Determines output format.
->Determines output title.

Last SELECT statement:
->Contains ORDER BY for the entire result, if required.

EXAMPLE:
SELECT first_name
,last_name
,'employee' (TITLE 'employee//type')
FROM employee
WHERE manager_employee_number = 1019
UNION
SELECT first_name
,last_name
,' manager '
FROM employee
WHERE employee_number = 1019
ORDER BY 2
;

RESULT:

first_name last_name employee_type
Carol Kanieski employee
Ron Kubic manager
John Stein employee

JOIN:
Join is a technique for accessing data from more than one
table in a single answerset. Each row in the answerset may
contain data from columns of more than one table. Tables
are joined on columns sharing common domain.

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