fix(mysql): qualify column reference in NULLS LAST CASE simulation

[brdbry]

Problem

When transpiling ORDER BY <col> from a NULL_ORDERING = "nulls_are_last" source dialect (e.g. DuckDB) into MySQL, the generator emits a CASE WHEN <col> IS NULL THEN 1 ELSE 0 END, <col> shim to simulate NULLS LAST (MySQL has no native NULLS FIRST/LAST syntax). The shim inlines the raw ORDER BY expression verbatim — typically a bare column name.

That's fine for a single-table query, but in a multi-table FROM where the same unqualified column name exists in more than one table, MySQL raises:

ERROR 1052 (23000): Column '<name>' in order clause is ambiguous

…even when there is an unambiguous SELECT-list projection. MySQL's ORDER BY does apply alias-first column resolution against the SELECT list — but inside the CASE expression in ORDER BY, that alias-first rule does not apply. The bare reference falls through to base-table resolution and collides.

Reproducer

from sqlglot import parse_one

src = (
    "SELECT e.employee_id FROM employees e "
    "LEFT JOIN employee_positions ep ON e.employee_id = ep.employee_id "
    "ORDER BY employee_id"
)
print(parse_one(src, read="duckdb").sql(dialect="mysql"))

Output today (rejected by MySQL 8.0):

SELECT e.employee_id FROM employees AS e LEFT JOIN employee_positions AS ep
  ON e.employee_id = ep.employee_id
ORDER BY CASE WHEN employee_id IS NULL THEN 1 ELSE 0 END, employee_id

After this PR:

SELECT e.employee_id FROM employees AS e LEFT JOIN employee_positions AS ep
  ON e.employee_id = ep.employee_id
ORDER BY CASE WHEN e.employee_id IS NULL THEN 1 ELSE 0 END, e.employee_id

Verified against a running MySQL 8.0 container: the pre-fix SQL errors with 1052, the post-fix SQL succeeds.

Fix

New helper Generator._qualified_for_null_ordering_simulation on the base generator walks from the Ordered node to its enclosing Select and looks for a single qualified-column projection whose output_name matches the bare ORDER BY column. If exactly one is found (whether the projection is plain e.employee_id or e.employee_id AS <alias>), the simulation substitutes that qualified column inside both the CASE and the trailing sort key. In every other case (already-qualified ORDER BY, positional/ordinal ORDER BY, expression ORDER BY, no enclosing Select, ambiguous projection match, inside a window function), the helper returns None and the existing behaviour is preserved.

Single change: sqlglot/generator.py.

Test

Added TestMySQL.test_null_ordering_simulation_qualifies_ambiguous_columns in tests/dialects/test_mysql.py covering three scenarios: the reproducer (multi-table LEFT JOIN, unqualified ORDER BY), an aliased projection (SELECT e.employee_id AS emp ... ORDER BY emp), and an already-qualified ORDER BY (no change in output).

Full suite passes locally (1107 passed, 18002 subtests passed).

Scope / out-of-scope

The fix lives in the base Generator.ordered_sql simulation branch, so any dialect with NULL_ORDERING_SUPPORTED is None (currently MySQL and MSSQL/TSQL) benefits in one place. I've only added a MySQL-focused test and PR title because the reported bug is specifically the MySQL 1052 symptom; happy to broaden the test coverage to TSQL or any other affected dialect in a follow-up if maintainers want it.

The fix is deliberately conservative: it only qualifies when a single qualified-column projection unambiguously matches, and never strips or rewrites already-qualified ORDER BY references. I did not modify the parser to track explicit-vs-implicit NULL ordering on Ordered nodes (broader ripple) or add any per-dialect toggle.

Context

Same area of the codebase as #6655 (UPDATE … FROM → UPDATE … JOIN translation) which I authored earlier this year.