create table t11 (a int not null);
insert into t11 values (0),(1),(2),(3);

create table t12 (pk int not null primary key, b int not null);
insert into t12 select a,a from t11;

explain select * from t11 left join t12 on t12.pk=t12.b and t12.b=3;
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2 rows in set (0.00 sec)

mysql> show warnings\G

• • • • • • • • • • • • • • • • • • • • • • • • • • • 1. row ***************************
                                                      Level: Note
                                                      Code: 1003
                                                      Message: select `j33`.`t11`.`a` AS `a`,`j33`.`t12`.`pk` AS `pk`,`j33`.`t12`.`b` AS `b` from `j33`.`t11` left join `j33`.`t12` on(((`j33`.`t12`.`pk` = `j33`.`t12`.`b`) and (`j33`.`t12`.`b` = 3))) where 1

mysql> explain extended select * from t11 left join t12 on t12.pk=t12.b and t12.b=3 where t11.a=333;
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2 rows in set, 1 warning (0.00 sec)

mysql> show warnings\G

• • • • • • • • • • • • • • • • • • • • • • • • • • • 1. row ***************************
                                                      Level: Note
                                                      Code: 1003
                                                      Message: select `j33`.`t11`.`a` AS `a`,`j33`.`t12`.`pk` AS `pk`,`j33`.`t12`.`b` AS `b` from `j33`.`t11` left join `j33`.`t12` on(((`j33`.`t12`.`pk` = 3) and (`j33`.`t12`.`b` = 3))) where (`j33`.`t11`.`a` = 333)
                                                      1 row in set (0.00 sec)