I’m writing a transpiler (of a custom programming language) into SQL. I’m representing tagged discriminated unions in DB by having all of typed columns and discriminator, which specified actual type at runtime (or in other words, which column to take value from – because others are null).
In my case, I need to return multiple columns + discriminator, thus I aggregate it into array on case expression. The snippet is for illustrational purposes (though working).
CREATE TYPE something AS (c1 INT, c2 TEXT, discriminator TEXT); WITH my_table AS (SELECT * FROM (VALUES (1), (2), (3)) t(value)) SELECT (unnesting.data).* FROM ( SELECT unnest(q.v) AS data FROM ( SELECT (CASE WHEN my_table.value = 1 THEN (SELECT array_agg(q0.* :: something) FROM (SELECT 111, NULL, 'Int') q0) ELSE (SELECT array_agg(q1.* :: something) FROM (SELECT NULL, 'myString', 'String') q1) END) AS v FROM my_table) q ) unnesting
The above code works, but it requires intermediate representation of
something. Is there a way to manually specify columns definition on select?
I know there is
ROWS FROM, but it only works with function, not with select, so I can’t do
... ROWS FROM (...) AS (INT, TEXT, TEXT)
Is there a way I could achieve similiar result?
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