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. TheINTERSECT and EXCEPT constructs resolve dissimilar types in the same way as UNION. Some other constructs, includingCASE, ARRAY, VALUES, and the GREATEST and LEASTfunctions, 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. [12]

  3. If all inputs are of type unknown, resolve as typetext (the preferred type of the string category). Otherwise, unknown inputs are ignored for the purposes of the remaining rules.

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

  5. Select the first non-unknown input type as the candidate type, then consider each other non-unknown input type, left to right. [13]If the candidate type can be implicitly converted to the other type, but not vice-versa, select the other type as the new candidate type. Then continue considering the remaining inputs. If, at any stage of this process, a preferred type is selected, stop considering additional inputs.

  6. Convert all inputs to the final candidate type. Fail if there is not an implicit conversion from a given input type to the candidate type.

Some examples follow.

Example 10.10. Type Resolution with Underspecified Types in a Union

SELECT text 'a' AS "text" UNION SELECT 'b';

 text