How to Code a UNION

A UNION combines the results of two or more queries into a single result set
Each result set must have the same number of columns
The corresponding data types for each column must be compatible. However, the column names may be different from each result set
A UNION removes duplicate rows by default
You may interfile the results using an ORDERY BY clause if there is a column with a common name.

Code Example:

1 USE world; 2 SELECT name, population 3 FROM city WHERE CountryCode = 'AUS' 4 UNION 5 SELECT name, population 6 FROM country 7 WHERE continent = 'Oceania' 8 ORDER BY name;

Results:

SELECT name, population
FROM city
WHERE CountryCode = 'AUS'

The first query returns the name and population from the city table.
The filter (WHERE CLAUSE) of the query limits the country code to Australia.

UNION

The ‘UNION’ clause will combine this query with the results of the subsequent query.

SELECT name, population
FROM country
WHERE continent = 'Oceania'

The second query returns the name and population from the country table.
The filter (WHERE CLAUSE) of the query limits the continent code to Oceania.

ORDER BY name;

It is possible to sort (ORDER BY CLAUSE) and interfile the results of both queries because each query shares a column with the same name. Otherwise, the ORDER BY clause would generate an error.