Add support for SELECT ON

clickhouse_sqlalchemy/drivers/compilers/sqlcompiler.py

@@ -498,3 +498,23 @@ def visit_not_ilike_op_binary(self, binary, operator, **kw):
             self.process(binary.left, **kw),
             self.process(binary.right, **kw)
         )
+
+    def get_select_precolumns(self, select, **kw):
+        # Do not call super().get_select_precolumns because
+        # it will warn/raise when distinct on is present
+        if select._distinct or select._distinct_on:
+            if select._distinct_on:
+                return (
+                    "DISTINCT ON ("
+                    + ", ".join(
+                        [
+                            self.process(col, **kw)
+                            for col in select._distinct_on
+                        ]
+                    )
+                    + ") "
+                )
+            else:
+                return "DISTINCT "
+        else:
+            return ""

tests/sql/test_selectable.py

@@ -559,3 +559,12 @@ def test_sql_expression_join(self):
             self.compile(join),
             'table_1 INNER JOIN table_2 ON table_1.x = table_2.x'
         )
+
+    def test_distinct_on(self):
+        table = self._make_table()
+
+        query = select(table.c.x).distinct(table.c.x)
+        self.assertEqual(
+            self.compile(query),
+            'SELECT DISTINCT ON (t1.x) t1.x FROM t1'
+        )