PostgreSQL 7.4 Documentation
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# 10.5. UNION, CASE, and ARRAY 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 and ARRAY constructs use the identical algorithm to match up their component expressions and select a result data type.

UNION, CASE, and ARRAY Type Resolution

1. 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.

2. If the non-unknown inputs are not all of the same type category, fail.

3. 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.

4. 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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