SQL UNION constructs must match up possibly dissimilar
types to become a single result set. The resolution algorithm is
applied separately to each output column of a union query. The
INTERSECT and EXCEPT constructs resolve
dissimilar types in the same way as UNION. The
LEAST constructs use the identical
algorithm to match up their component expressions and select a result
Type Resolution for UNION, CASE, and Related Constructs
If all inputs are of type unknown, resolve as type text (the preferred type of the string category). Otherwise, ignore the unknown inputs while choosing the result type.
If the non-unknown inputs are not all of the same type category, fail.
Choose the first non-unknown input type which is a preferred type in that category or allows all the non-unknown inputs to be implicitly converted to it.
Convert all inputs to the selected type.
Some examples follow.
Example 10-7. Type Resolution with Underspecified Types in a Union
SELECT text 'a' AS "text" UNION SELECT 'b'; text ------ a b (2 rows)
Here, the unknown-type literal 'b' will be resolved as type text.
Example 10-8. Type Resolution in a Simple Union
SELECT 1.2 AS "numeric" UNION SELECT 1; numeric --------- 1 1.2 (2 rows)
The literal 1.2 is of type numeric, and the integer value 1 can be cast implicitly to numeric, so that type is used.