Union
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Union Data Type

A UNION type (not to be confused with the SQL UNION operator) is a nested type capable of holding one of multiple “alternative” values, much like the union in C. The main difference being that these UNION types are tagged unions and thus always carry a discriminator “tag” which signals which alternative it is currently holding, even if the inner value itself is null. UNION types are thus more similar to C++17’s std::variant, Rust’s Enum or the “sum type” present in most functional languages.

UNION types must always have at least one member, and while they can contain multiple members of the same type, the tag names must be unique. UNION types can have at most 256 members.

Under the hood, UNION types are implemented on top of STRUCT types, and simply keep the “tag” as the first entry.

UNION values can be created with the UNION_VALUE(tag := expr) function or by casting from a member type.

Example

-- Create a table with a union column
CREATE TABLE tbl1(u UNION(num INT, str VARCHAR));

-- Any type can be implicitly cast to a union containing the type.
-- Any union can also be implicitly cast to a another union if 
-- the source union members are a subset of the targets.
-- Note: only if the cast is unambiguous! 
-- More details in the 'Union casts' section below.
INSERT INTO tbl1 values (1) , ('two') , (union_value(str := 'three'));
-- Union use the member types varchar cast functions when casting to varchar.
SELECT u from tbl1;
-- returns:
--    1
--    two
--    three
-- Select all the 'str' members
SELECT union_extract(u, 'str') FROM tbl1;
-- Alternatively, you can use 'dot syntax' like with structs
SELECT u.str FROM tbl1;
-- returns: 
--    NULL
--    two
--    three

-- Select the currently active tag from the union as an enum.
SELECT union_tag(u) FROM tbl1;
-- returns:
--    num
--    str
--    str

Union casts

Compared to other nested types, UNIONs allow a set of implicit casts to facilitate unintrusive and natural usage when working with their members as “subtypes”. However, these casts have been designed with two principles in mind, to avoid ambiguity and to avoid casts that could lead to loss of information. This prevents UNIONs from being completely “transparent”, while still allowing UNION types to have a “supertype” relationship with their members.

Thus UNION types can’t be implicitly cast to any of their member types in general, since the information in the other members not matching the target type would be “lost”. If you want to coerce a UNION into one of its members, you should use the union_extract function explicitly instead.

The only exception to this is when casting a UNION to VARCHAR, in which case the members will all use their corresponding VARCHAR casts. Since everything can be cast to VARCHAR, this is “safe” in a sense.

Casting to unions

A type can always be implicitly cast to a UNION if it can be implicitly cast to one of the UNION member types.

  • If there are multiple candidates, the built in implicit casting priority rules determine the target type. For example, a FLOAT -> UNION(i INT, v VARCHAR) cast will always cast the FLOAT to the INT member before VARCHAR.
  • If the cast still is ambiguous, i.e. there are multiple candidates with the same implicit casting priority, an error is raised. This usually happens when the UNION contains multiple members of the same type, e.g. a FLOAT -> UNION(i INT, num INT) is always ambiguous.

So how do we disambiguate if we want to create a UNION with multiple members of the same type? By using the union_value function, which takes a keyword argument specifying the tag. For example, union_value(num := 2::INT) will create a UNION with a single member of type INT with the tag num. This can then be used to disambiguate in an explicit (or implicit, read on below!) UNION to UNION cast, like CAST(union_value(b := 2) AS UNION(a INT, b INT)).

Casting between unions

UNION types can be cast between each other if the source type is a “subset” of the target type. In other words, all the tags in the source UNION must be present in the target UNION, and all the types of the matching tags must be implicitly castable between source and target. In essence, this means that UNION types are covariant with respect to their members.

Ok Source Target Comments
UNION(a A, b B) UNION(a A, b B, c C)  
UNION(a A, b B) UNION(a A, b C) if B can be implicitly cast to C
UNION(a A, b B, c C) UNION(a A, b B)  
UNION(a A, b B) UNION(a A, b C) if B can’t be implicitly cast to C
UNION(A, B, D) UNION(A, B, C)  

Comparison and Sorting

Since UNION types are implemented on top of STRUCT types internally, they can be used with all the comparison operators as well as in both WHERE and HAVING clauses with the same semantics as STRUCTs. The “tag” is always stored as the first struct entry, which ensures that the UNION types are compared and ordered by “tag” first.

Functions

See Nested Functions.

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