L025: Derived query requires alias -- also handle UNION, etc.

src/sqlfluff/rules/L025.py

@@ -121,7 +121,9 @@ def is_alias_required(
                         dialect_name in cls._dialects_requiring_alias_for_values_clause
                     )
                 elif any(
-                    seg.is_type("select_statement")
+                    seg.is_type(
+                        "select_statement", "set_expression", "with_compound_statement"
+                    )
                     for seg in segment.iter_segments(expanding=("bracketed",))
                 ):
                     # The FROM expression is a derived table, i.e. a nested

test/fixtures/rules/std_rule_cases/L025.yml

@@ -312,8 +312,28 @@ test_fail_snowflake_flatten_function:
     core:
       dialect: snowflake
 
-test_pass_derived_query_requires_alias:
+test_pass_derived_query_requires_alias_1:
+  # Case 1: Simple derived query
   pass_str: |
     SELECT * FROM (
         SELECT 1
     )  as a
+
+test_pass_derived_query_requires_alias_2:
+  # Case 2: Derived query uses set operation (UNION)
+  pass_str: |
+    SELECT * FROM (
+        SELECT col FROM dbo.tab
+        UNION
+        SELECT -1 AS col
+    ) AS a
+
+test_pass_derived_query_requires_alias_3:
+  # Case 3: Derived query includes a WITH statement
+  pass_str: |
+    SELECT * FROM (
+        WITH foo AS (
+            SELECT col FROM dbo.tab
+        )
+        SELECT * FROM foo
+    ) AS a