PostgreSQL 9.6.6 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

1. If all inputs are of the same type, and it is not `unknown`, resolve as that type.

2. If any input is of a domain type, treat it as being of the domain's base type for all subsequent steps. [1]

3. If all inputs are of type `unknown`, resolve as type `text` (the preferred type of the string category). Otherwise, `unknown` inputs are ignored.

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

5. Choose the first non-unknown input type which is a preferred type in that category, if there is one.

6. Otherwise, choose the last non-unknown input type that allows all the preceding non-unknown inputs to be implicitly converted to it. (There always is such a type, since at least the first type in the list must satisfy this condition.)

7. Convert all inputs to the selected type. Fail if there is not a conversion from a given input to the selected type.

Some examples follow.

Example 10-9. 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 to type `text`.

Example 10-10. 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 10-11. 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`.

### Notes

[1] | Somewhat like the treatment of domain inputs for operators and functions, this behavior allows a domain type to be preserved through a `UNION` or similar construct, so long as the user is careful to ensure that all inputs are implicitly or explicitly of that exact type. Otherwise the domain's base type will be preferred.

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