UNION, INTERSECT, EXCEPT)The results of two queries can be combined using the set operations union, intersection, and difference. The syntax is
query1UNION [ALL]query2query1INTERSECT [ALL]query2query1EXCEPT [ALL]query2
   where query1 and
   query2 are queries that can use any of
   the features discussed up to this point.
  
   UNION effectively appends the result of
   query2 to the result of
   query1 (although there is no guarantee
   that this is the order in which the rows are actually returned).
   Furthermore, it eliminates duplicate rows from its result, in the same
   way as DISTINCT, unless UNION ALL is used.
  
   INTERSECT returns all rows that are both in the result
   of query1 and in the result of
   query2.  Duplicate rows are eliminated
   unless INTERSECT ALL is used.
  
   EXCEPT returns all rows that are in the result of
   query1 but not in the result of
   query2.  (This is sometimes called the
   difference between two queries.)  Again, duplicates
   are eliminated unless EXCEPT ALL is used.
  
In order to calculate the union, intersection, or difference of two queries, the two queries must be “union compatible”, which means that they return the same number of columns and the corresponding columns have compatible data types, as described in Section 10.5.
Set operations can be combined, for example
query1UNIONquery2EXCEPTquery3
which is equivalent to
(query1UNIONquery2) EXCEPTquery3
   As shown here, you can use parentheses to control the order of
   evaluation.  Without parentheses, UNION
   and EXCEPT associate left-to-right,
   but INTERSECT binds more tightly than those two
   operators.  Thus
query1UNIONquery2INTERSECTquery3
means
query1UNION (query2INTERSECTquery3)
   You can also surround an individual query
   with parentheses.  This is important if
   the query needs to use any of the clauses
   discussed in following sections, such as LIMIT.
   Without parentheses, you'll get a syntax error, or else the clause will
   be understood as applying to the output of the set operation rather
   than one of its inputs.  For example,
SELECT a FROM b UNION SELECT x FROM y LIMIT 10
is accepted, but it means
(SELECT a FROM b UNION SELECT x FROM y) LIMIT 10
not
SELECT a FROM b UNION (SELECT x FROM y LIMIT 10)