``++UNION++`` is a convenient syntax to combine the results of two or more SQL statements because it helps you cut a complex problem into multiple simple SQL statements. But when it comes to execution, using ``++UNION++`` is debatable. 


First, it may be possible to fuse two simple SQL statements into a bigger one that will run faster. Second, ``++UNION++`` is significantly less performant compared to ``++UNION ALL++`` because it removes duplicated entries and runS an internal ``++DISTINCT++`` to achieve this.


``++UNION ALL++`` does not remove duplicates and returns all the results from the queries. It performs faster in most cases compared to ``++UNION++``. Nevertheless, the quantity of data returned by ``++UNION ALL++`` can be significantly larger than with ``++UNION++``. On a slow network, the performance gain of using ``++UNION ALL++`` instead of ``++UNION++`` can be negated by the time lost in the larger data transfer.


This rule raises an issue on each ``++UNION++``. It's up to the developer to challenge its use and see if there is a better way to rewrite without ``++UNION++``.

== Noncompliant Code Example

----
-- case #1
SELECT EMAIL FROM EMPLOYEES WHERE COUNTRY = 'FR'
UNION                           -- Noncompliant
SELECT EMAIL FROM EMPLOYEES WHERE COUNTRY = 'CH'

-- case #2
-- if you care about not having duplicated entries, then UNION is the good choice
SELECT EMAIL FROM EMPLOYEES 
UNION                           -- Noncompliant
SELECT EMAIL FROM CUSTOMERS
----

== Compliant Solution

----
-- case #1
SELECT EMAIL FROM EMPLOYEES WHERE COUNTRY = 'FR' OR COUNTRY = 'CH'

-- case #2
-- if you don't care about duplicated entries in the results of this UNION, then UNION ALL should be preferred
SELECT EMAIL FROM EMPLOYEES 
UNION ALL
SELECT EMAIL FROM CUSTOMERS
----
