$structVariantType = Variant<a: Int32, b: String>;

$data = [
    <|aValue: 1,  bValue: "world"|>,
    <|aValue: 42, bValue: "moon"|>,
    <|aValue: 7,  bValue: "hello"|>,
    <|aValue: 99, bValue: "yql"|>,
];

SELECT
    Variant(aValue, "a", $structVariantType) AS aAlternative,
    Variant(bValue, "b", $structVariantType) AS bAlternative
FROM as_table($data);