UNION, CASE, and Related Constructs

PostgreSQL 9.6.3 Documentation
Prev | Up | Chapter 10. Type Conversion | Next

# 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.

Prev | Home | Next
Value Storage | Up | Indexes