with cte as

(select a from t1 where a between 4 and 7 group by a)

(select a from cte where exists( select a from t1 where cte.a=t1.a )) 

union

(select a from t1 where a < 2);

with cte as

(select a from t1 where a between 4 and 7 group by a)

(select a from t1 where a < 2)

union

(select a from cte where exists( select a from t1 where cte.a=t1.a )); 

MariaDB [test]> create table t1 (a int);

 MariaDB [test]> insert into t1 values

    ->   (7), (2), (8), (1), (3), (2), (7), (5), (4), (7), (9), (8);

MariaDB [test]> prepare stmt from "

with cte as

(select a from t1 where a between 4 and 7 group by a)

(select a from cte where exists( select a from t1 where cte.a=t1.a )) 

union

(select a from t1 where a < 2); 

";

Statement prepared

 MariaDB [test]> execute stmt;      

+------+

| a    |

+------+

|    7 |

|    5 |

|    4 |

|    1 |

+------+

MariaDB [test]> execute stmt;

+------+

| a    |

+------+

|    7 |

|    5 |

|    4 |

|    1 |

+------+

MariaDB [test]> prepare stmt from "

    "> with cte as

    "> (select a from t1 where a between 4 and 7 group by a)

    "> (select a from t1 where a < 2)

    "> union

    "> (select a from cte where exists( select a from t1 where cte.a=t1.a )) 

    "> ";

Statement prepared

MariaDB [test]> execute stmt;

mysqld: /home/igor/maria-git/10.2/sql/sql_prepare.cc:2949: void reinit_stmt_before_use(THD*, LEX*): Assertion `sl->join == 0' failed.