feat(snowflake): transpile CORR with NaN-->NULL

[treysp]

CORR performs an internal calculation that has VARIANCE([column]) in the denominator. When that occurs, Snowflake returns NULL while DuckDB returns NaN.

To replicate the Snowflake's NULL in DuckDB, we must wrap the CORR call in ISNAN:

CASE WHEN ISNAN(CORR(a, b)) THEN NULL ELSE CORR(a, b) END

Naively wrapping in ISNAN generates invalid syntax for windowed and filtered function calls.

Therefore, we add special DuckDB generator handling to route exp.Window, exp.Window(exp.Filter), and exp.Filter to a corr_sql method designed to accept window and filtered expressions.

Examples:

• Snowflake: SELECT CORR(a, b) OVER (PARTITION BY c)
  • DuckDB: SELECT CASE WHEN ISNAN(CORR(a, b) OVER (PARTITION BY c)) THEN NULL ELSE CORR(a, b) OVER (PARTITION BY c) END
• Snowflake: SELECT CORR(a, b) FILTER (WHERE c > 0)
  • DuckDB: SELECT CASE WHEN ISNAN(CORR(a, b) FILTER(WHERE c > 0)) THEN NULL ELSE CORR(a, b) FILTER(WHERE c > 0) END
• Snowflake: SELECT CORR(a, b) FILTER (WHERE c > 0) OVER (PARTITION BY d)
  • DuckDB: SELECT CASE WHEN ISNAN(CORR(a, b) FILTER(WHERE c > 0) OVER (PARTITION BY d)) THEN NULL ELSE CORR(a, b) FILTER(WHERE c > 0) OVER (PARTITION BY d) END

[github-actions]

SQLGlot Integration Test Results

Comparing:

• this branch (sqlglot:trey/corr-sf-duckdb, sqlglot version: trey/corr-sf-duckdb)
• baseline (main, sqlglot version: 28.5.1.dev72)

⚠️ Limited to dialects: snowflake, duckdb

By Dialect

dialect	main	sqlglot:trey/corr-sf-duckdb	difference	links
duckdb -> duckdb	4003/4003 passed (100.0%)	4003/4003 passed (100.0%)	No change	full result / delta
snowflake -> duckdb	578/847 passed (68.2%)	578/847 passed (68.2%)	No change	full result / delta
snowflake -> snowflake	847/847 passed (100.0%)	847/847 passed (100.0%)	No change	full result / delta

Overall

main: 5697 total, 5428 passed (pass rate: 95.3%), sqlglot version: 28.5.1.dev72

sqlglot:trey/corr-sf-duckdb: 5697 total, 5428 passed (pass rate: 95.3%), sqlglot version: trey/corr-sf-duckdb

Difference: No change

[georgesittas]

@treysp let me know if there's anything blocking this.