OQL is for persistable entities only.

Author: MarkvanMents

content/en/docs/refguide/modeling/domain-model/oql/_index.md

@@ -10,6 +10,10 @@ The Mendix Object Query Language (OQL) is a relational query language inspired b
 
 In addition, OQL can use predefined relations (associations) to easily join objects without having to calculate which columns should be coupled. Despite these differences, many SQL keywords also work in OQL.
 
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+OQL can only be used on persistable entities and the associations between them. It cannot be used to reference non-persistable, external, or view [entities](/refguide/entities/). 
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 Some examples of OQL queries are:
 
 * `SELECT LastName FROM Sales.Customer` – retrieves the family names of all customers