union and or - sqlsql supports union explicitly

Question

UNION and OR - SQL

SQL supports UNION explicitly but differently from the way it supports JOIN explicitly. As we have seen, JOIN is used exclusively within the FROM clause, such that IS_CALLED NATURAL JOIN IS_ENROLLED_ON, for example, can be an element of that clause but cannot stand alone as a table expression. Instead, UNION always connects table expressions that can stand alone, these being:

1. SELECT expressions
2. TABLE tn, which is equivalent to SELECT * FROM tn, where tn is a table name
3. VALUES expressions
4. Invocations of UNION, INTERSECT and EXCEPT