Unparse inner join with no conditions as a cross join

datafusion/sql/src/unparser/plan.rs

@@ -876,7 +876,16 @@ impl Unparser<'_> {
         constraint: ast::JoinConstraint,
     ) -> Result<ast::JoinOperator> {
         Ok(match join_type {
-            JoinType::Inner => ast::JoinOperator::Inner(constraint),
+            JoinType::Inner => match &constraint {
+                ast::JoinConstraint::On(_)
+                | ast::JoinConstraint::Using(_)
+                | ast::JoinConstraint::Natural => ast::JoinOperator::Inner(constraint),
+                ast::JoinConstraint::None => {
+                    // Inner joins with no conditions or filters are not valid SQL in most systems,
+                    // return a CROSS JOIN instead
+                    ast::JoinOperator::CrossJoin
+                }
+            },
             JoinType::Left => ast::JoinOperator::LeftOuter(constraint),
             JoinType::Right => ast::JoinOperator::RightOuter(constraint),
             JoinType::Full => ast::JoinOperator::FullOuter(constraint),

datafusion/sql/tests/cases/plan_to_sql.rs

@@ -296,7 +296,7 @@ fn roundtrip_statement_with_dialect() -> Result<()> {
         TestStatementWithDialect {
             sql: "select min(ta.j1_id) as j1_min, max(tb.j1_max) from j1 ta, (select distinct max(ta.j1_id) as j1_max from j1 ta order by max(ta.j1_id)) tb order by min(ta.j1_id) limit 10;",
             expected:
-                "SELECT `j1_min`, `max(tb.j1_max)` FROM (SELECT min(`ta`.`j1_id`) AS `j1_min`, max(`tb`.`j1_max`), min(`ta`.`j1_id`) FROM `j1` AS `ta` JOIN (SELECT `j1_max` FROM (SELECT DISTINCT max(`ta`.`j1_id`) AS `j1_max` FROM `j1` AS `ta`) AS `derived_distinct`) AS `tb` ORDER BY min(`ta`.`j1_id`) ASC) AS `derived_sort` LIMIT 10",
+                "SELECT `j1_min`, `max(tb.j1_max)` FROM (SELECT min(`ta`.`j1_id`) AS `j1_min`, max(`tb`.`j1_max`), min(`ta`.`j1_id`) FROM `j1` AS `ta` CROSS JOIN (SELECT `j1_max` FROM (SELECT DISTINCT max(`ta`.`j1_id`) AS `j1_max` FROM `j1` AS `ta`) AS `derived_distinct`) AS `tb` ORDER BY min(`ta`.`j1_id`) ASC) AS `derived_sort` LIMIT 10",
             parser_dialect: Box::new(MySqlDialect {}),
             unparser_dialect: Box::new(UnparserMySqlDialect {}),
         },
@@ -1253,3 +1253,17 @@ fn test_unnest_to_sql() {
         r#"SELECT UNNEST([1, 2, 2, 5, NULL]) AS u1"#,
     );
 }
+
+#[test]
+fn test_join_with_no_conditions() {
+    sql_round_trip(
+        GenericDialect {},
+        "SELECT * FROM j1 JOIN j2",
+        "SELECT * FROM j1 CROSS JOIN j2",
+    );
+    sql_round_trip(
+        GenericDialect {},
+        "SELECT * FROM j1 CROSS JOIN j2",
+        "SELECT * FROM j1 CROSS JOIN j2",
+    );
+}