fix(mysql): improve update SQL generation for MySql

Author: brdbry

sqlglot/dialects/mysql.py

@@ -1340,15 +1340,24 @@ def _update_from_joins_sql(self, expression: exp.Update) -> t.Tuple[str, str]:
             if not from_expr:
                 return ("", "")
 
-            table = from_expr.this.copy()
+            # Qualify unqualified columns in SET clause with the target table
+            # MySQL requires qualified column names in multi-table UPDATE to avoid ambiguity
+            target_table = expression.this
+            if isinstance(target_table, exp.Table):
+                target_name = target_table.alias_or_name
+                for eq in expression.expressions:
+                    col = eq.this
+                    if isinstance(col, exp.Column) and not col.table:
+                        col.set("table", exp.to_identifier(target_name))
+
+            table = from_expr.this
             nested_joins = table.args.get("joins") or []
             if nested_joins:
                 table.set("joins", None)
 
             join_sql = self.sql(exp.Join(this=table, on=exp.true()))
             for nested in nested_joins:
                 if not nested.args.get("on") and not nested.args.get("using"):
-                    nested = nested.copy()
                     nested.set("on", exp.true())
                 join_sql += self.sql(nested)
 

sqlglot/generator.py

@@ -2234,8 +2234,8 @@ def _update_from_joins_sql(self, expression: exp.Update) -> t.Tuple[str, str]:
 
     def update_sql(self, expression: exp.Update) -> str:
         this = self.sql(expression, "this")
-        set_sql = self.expressions(expression, flat=True)
         join_sql, from_sql = self._update_from_joins_sql(expression)
+        set_sql = self.expressions(expression, flat=True)
         where_sql = self.sql(expression, "where")
         returning = self.sql(expression, "returning")
         order = self.sql(expression, "order")

tests/dialects/test_mysql.py

@@ -195,11 +195,28 @@ def test_ddl(self):
         self.validate_identity("ALTER TABLE t ALTER COLUMN c SET VISIBLE")
 
     def test_update_from_to_join(self):
+        # MySQL multi-table UPDATE requires qualified columns in SET to avoid ambiguity
         self.validate_all(
-            "UPDATE foo JOIN bar ON TRUE SET a = bar.a WHERE foo.id = bar.id",
+            "UPDATE foo JOIN bar ON TRUE SET foo.a = bar.a WHERE foo.id = bar.id",
             read={
                 "postgres": "UPDATE foo SET a = bar.a FROM bar WHERE foo.id = bar.id",
-                "mysql": "UPDATE foo JOIN bar ON TRUE SET a = bar.a WHERE foo.id = bar.id",
+                "mysql": "UPDATE foo JOIN bar ON TRUE SET foo.a = bar.a WHERE foo.id = bar.id",
+            },
+        )
+
+        # Multiple columns in SET clause
+        self.validate_all(
+            "UPDATE t1 JOIN t2 ON TRUE SET t1.id = t2.id, t1.name = t2.name WHERE t1.x = t2.x",
+            read={
+                "postgres": "UPDATE t1 SET id = t2.id, name = t2.name FROM t2 WHERE t1.x = t2.x",
+            },
+        )
+
+        # Already qualified columns in Postgres should remain qualified
+        self.validate_all(
+            "UPDATE t1 JOIN t2 ON TRUE SET t1.id = t2.id WHERE t1.x = t2.x",
+            read={
+                "postgres": "UPDATE t1 SET t1.id = t2.id FROM t2 WHERE t1.x = t2.x",
             },
         )
 

tests/dialects/test_teradata.py

@@ -70,7 +70,7 @@ def test_update(self):
             "UPDATE A FROM schema.tableA AS A, (SELECT col1 FROM schema.tableA GROUP BY col1) AS B SET col2 = '' WHERE A.col1 = B.col1",
             write={
                 "teradata": "UPDATE A FROM schema.tableA AS A, (SELECT col1 FROM schema.tableA GROUP BY col1) AS B SET col2 = '' WHERE A.col1 = B.col1",
-                "mysql": "UPDATE A JOIN `schema`.tableA AS A ON TRUE JOIN (SELECT col1 FROM `schema`.tableA GROUP BY col1) AS B ON TRUE SET col2 = '' WHERE A.col1 = B.col1",
+                "mysql": "UPDATE A JOIN `schema`.tableA AS A ON TRUE JOIN (SELECT col1 FROM `schema`.tableA GROUP BY col1) AS B ON TRUE SET A.col2 = '' WHERE A.col1 = B.col1",
             },
         )