---
title: "`UNION`, `CASE`, and Related Constructs"
id: typeconv-union-case
pg_version: "20devel"
---

<a id="typeconv-union-case"></a>

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

Type Resolution for `UNION`, `CASE`, and Related Constructs1. 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. [13] 


3. If all inputs are of type `unknown`, resolve as type `text` (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. [14] 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. 

**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`. 

**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. 

**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`. 

**Type Resolution in a Nested Union**


SELECT NULL UNION SELECT NULL UNION SELECT 1;

ERROR:  UNION types text and integer cannot be matched
 This failure occurs because PostgreSQL treats multiple `UNION`s as a nest of pairwise operations; that is, this input is the same as 
(SELECT NULL UNION SELECT NULL) UNION SELECT 1;
 The inner `UNION` is resolved as emitting type `text`, according to the rules given above. Then the outer `UNION` has inputs of types `text` and `integer`, leading to the observed error. The problem can be fixed by ensuring that the leftmost `UNION` has at least one input of the desired result type. 

`INTERSECT` and `EXCEPT` operations are likewise resolved pairwise. However, the other constructs described in this section consider all of their inputs in one resolution step. 

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 used. 

For historical reasons, `CASE` treats its `ELSE` clause (if any) as the "first" input, with the `THEN` clauses(s) considered after that. In all other cases, "left to right" means the order in which the expressions appear in the query text.