create table t1 (t1a int, t1b int, t1c int) engine=myisam;

insert into t1 values (1,1,1),(2,2,2);

create table t2 (t2a int, t2b int, t2c int) engine=myisam;

insert into t2 values (1,1,1),(2,2,2),(3,3,3);

MariaDB [test]> select * from t2 where t2a in ( select t1a from t1 group by (select t1a > t2b) order by t1a );

+------+------+------+

| t2a  | t2b  | t2c  |

+------+------+------+

|    1 |    1 |    1 |

|    2 |    2 |    2 |

+------+------+------+

2 rows in set (0.051 sec)

MariaDB [test]> select * from t2 where t2a in ( select t1a from t1 group by (select t1a > t2b where true) order by t1a );

+------+------+------+

| t2a  | t2b  | t2c  |

+------+------+------+

|    1 |    1 |    1 |

+------+------+------+

1 row in set (0.019 sec)

MariaDB [test]> prepare s from 'select * from t2 where t2a in ( select t1a from t1 group by (select t1a > t2b) order by t1a )';

Query OK, 0 rows affected (0.006 sec)

Statement prepared

MariaDB [test]> execute s;

+------+------+------+

| t2a  | t2b  | t2c  |

+------+------+------+

|    1 |    1 |    1 |

+------+------+------+