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MySQL Unions

Remarks:

UNION DISTINCT is the same as UNION; it is slower than UNION ALL because of a de-duplicating pass. A good practice is to always spell out DISTINCT or ALL, thereby signaling that you thought about which to do.

Union operator

The UNION operator is used to combine the result-set (only distinct values) of two or more SELECT statements.

Query: (To selects all the different cities (only distinct values) from the "Customers" and the "Suppliers" tables)

SELECT City FROM Customers
UNION
SELECT City FROM Suppliers
ORDER BY City;

Result:

Number of Records: 10

City
------
Aachen
Albuquerque
Anchorage
Annecy
Barcelona
Barquisimeto
Bend
Bergamo
Berlin
Bern

Union ALL

UNION ALL to select all (duplicate values also) cities from the "Customers" and "Suppliers" tables.

Query:

SELECT City FROM Customers
UNION ALL
SELECT City FROM Suppliers
ORDER BY City;

Result:

Number of Records: 12

City
-------
Aachen
Albuquerque
Anchorage
Ann Arbor
Annecy
Barcelona
Barquisimeto
Bend
Bergamo
Berlin
Berlin
Bern

UNION ALL With WHERE

UNION ALL to select all(duplicate values also) German cities from the "Customers" and "Suppliers" tables. Here Country="Germany" is to be specified in the where clause.

Query:

SELECT City, Country FROM Customers
WHERE Country='Germany'
UNION ALL
SELECT City, Country FROM Suppliers
WHERE Country='Germany'
ORDER BY City;

Result:

Number of Records: 14

City	Country
Aachen	Germany
Berlin	Germany
Berlin	Germany
Brandenburg	Germany
Cunewalde	Germany
Cuxhaven	Germany
Frankfurt	Germany
Frankfurt a.M.	Germany
Köln	Germany
Leipzig	Germany
Mannheim	Germany
München	Germany
Münster	Germany
Stuttgart	Germany

Syntax:

SELECT column_name(s) FROM table1 UNION SELECT column_name(s) FROM table2;
SELECT column_name(s) FROM table1 UNION ALL SELECT column_name(s) FROM table2;
SELECT column_name(s) FROM table1 WHERE col_name="XYZ" UNION ALL SELECT column_name(s) FROM table2 WHERE col_name="XYZ";

Contributors

Topic Id: 5376

Example Ids: 19139,19140,19141

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