PostgreSQL 8.2.5 Documentation
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10.5. `UNION`, `CASE`, and Related Constructs

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 CASE, ARRAY, VALUES, GREATEST and LEAST constructs use the identical algorithm to match up their component expressions and select a result data type.

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.

Example 10-9. Type Resolution in a Transposed Union

SELECT 1 AS "real" UNION SELECT CAST('2.2' AS REAL);

 real
------
    1
  2.2
(2 rows)

Here, since type real cannot be implicitly cast to integer, but integer can be implicitly cast to real, the union result type is resolved as real.

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10.5. UNION, CASE, and Related Constructs

10.5. `UNION`, `CASE`, and Related Constructs