fix(dialect): emit FROM UNNEST for FROM-less LATERAL VIEW EXPLODE (#8495)

[inchang-ing]

Summary

Spark allows a LATERAL VIEW EXPLODE without a FROM clause, e.g.:

SELECT n LATERAL VIEW EXPLODE(SEQUENCE(1, 3)) AS n

When transpiling to Presto/Trino/DuckDB, explode_to_unnest_sql turned this into a CROSS JOIN UNNEST(...), but with no FROM relation there is nothing on the left of the CROSS JOIN, producing invalid SQL:

SELECT n CROSS JOIN UNNEST(SEQUENCE(1, 3)) AS _t0(n)

Root cause

explode_to_unnest_sql (in sqlglot/dialects/dialect.py) always wraps the UNNEST in a CROSS JOIN. A CROSS JOIN requires a left relation, which doesn't exist when the parent SELECT has no FROM clause.

Fix

When the parent SELECT has no FROM, emit the UNNEST as the FROM clause instead:

SELECT n FROM UNNEST(SEQUENCE(1, 3)) AS _t0(n)

The existing FROM ... CROSS JOIN UNNEST(...) path (used when a FROM is present) is unchanged.

Verification

Added a regression test in tests/dialects/test_presto.py covering the spark → presto/trino/duckdb transpilation. All test_presto, test_trino, and test_spark suites pass.

Fixes #8495

[georgesittas]

Thanks, but handling this is out of scope for sqlglot. See linked issue.