support transpilation of GREATEST from BigQuery to DuckDB

Author: fivetran-felixhuang

sqlglot/dialects/duckdb.py

@@ -290,6 +290,37 @@ def _anyvalue_sql(self: DuckDB.Generator, expression: exp.AnyValue) -> str:
     return self.function_fallback_sql(expression)
 
 
+def _greatest_sql(self: DuckDB.Generator, expression: exp.Greatest) -> str:
+    """
+    Handle GREATEST function to match BigQuery's NULL and NaN behavior.
+    In BigQuery:
+    - if any argument is NULL, the result is NULL
+    In DuckDB:
+    - NULLs are ignored, returns greatest non-NULL value.
+
+    If any argument is NaN (and other arugments are not NULL), DuckDB returns NaN, which matches BigQuery's behavior
+    """
+    all_args = [expression.this] + expression.expressions
+
+    greatest_expr = self.func("GREATEST", *all_args)
+
+    # Check for NULL: arg1 IS NULL OR arg2 IS NULL OR ...
+    null_checks = []
+    for arg in all_args:
+        null_checks.append(exp.Is(this=arg, expression=exp.Null()))
+
+    is_any_null_expr: exp.Expression = null_checks[0]
+    for null_check in null_checks[1:]:
+        is_any_null_expr = exp.Or(this=is_any_null_expr, expression=null_check)
+
+    # Create CASE expression - only need NULL check, DuckDB handles NaN natively
+    case_expr = exp.Case(
+        ifs=[exp.If(this=is_any_null_expr, true=exp.Null())], default=greatest_expr
+    )
+
+    return self.sql(case_expr)
+
+
 class DuckDB(Dialect):
     NULL_ORDERING = "nulls_are_last"
     SUPPORTS_USER_DEFINED_TYPES = True
@@ -751,6 +782,7 @@ class Generator(generator.Generator):
             exp.EuclideanDistance: rename_func("LIST_DISTANCE"),
             exp.GenerateDateArray: _generate_datetime_array_sql,
             exp.GenerateTimestampArray: _generate_datetime_array_sql,
+            exp.Greatest: _greatest_sql,
             exp.GroupConcat: lambda self, e: groupconcat_sql(self, e, within_group=False),
             exp.Explode: rename_func("UNNEST"),
             exp.IntDiv: lambda self, e: self.binary(e, "//"),