Friday, November 13, 2015

Monday, October 19, 2015

The SQL UNION

The SQL UNION Operator

The UNION operator is used to combine the result-set of two or more SELECT statements.
Notice that each SELECT statement within the UNION must have the same number of columns. The columns must also have similar data types. Also, the columns in each SELECT statement must be in the same order.

SQL UNION Syntax

SELECT column_name(s) FROM table1
UNION
SELECT column_name(s) FROM table2;
 
Note: The UNION operator selects only distinct values by default. To allow duplicate values, use the ALL keyword with UNION.

SQL UNION ALL Syntax

SELECT column_name(s) FROM table1
UNION ALL
SELECT column_name(s) FROM table2;
 
PS: The column names in the result-set of a UNION are usually equal to the column names in the first SELECT statement in the UNION.

Demo Database

In this tutorial we will use the well-known Northwind sample database.
Below is a selection from the "Customers" table:
CustomerID CustomerName ContactName Address City PostalCode Country
1
Alfreds Futterkiste Maria Anders Obere Str. 57 Berlin 12209 Germany
2 Ana Trujillo Emparedados y helados Ana Trujillo Avda. de la Constitución 2222 México D.F. 05021 Mexico
3 Antonio Moreno Taquería Antonio Moreno Mataderos 2312 México D.F. 05023 Mexico
And a selection from the "Suppliers" table:
SupplierID SupplierName ContactName Address City PostalCode Country
1 Exotic Liquid Charlotte Cooper 49 Gilbert St. Londona EC1 4SD UK
2 New Orleans Cajun Delights Shelley Burke P.O. Box 78934 New Orleans 70117 USA
3 Grandma Kelly's Homestead Regina Murphy 707 Oxford Rd. Ann Arbor 48104 USA

SQL UNION Example

The following SQL statement selects all the different cities (only distinct values) from the "Customers" and the "Suppliers" tables:

Example

SELECT City FROM Customers
UNION
SELECT City FROM Suppliers
ORDER BY City; 
 
City
Aachen 
Albuquerque 
Anchorage 
Ann Arbor 
Annecy 
Århus 
Barcelona