выводим записи из таблицы reply
	select 
		id_reply_pk	
	from 
		reply
	where 
		reply.id_test_fk in ( 
	select 
		id_test_pk 
	from 
		tests
	where 
        tests.id_test_pk in (
	select 
		id_quest_pk 
	from
		quest
	where 
        quest.id_theme_fk = ( 
    select 
        id_theme_pk 
	from 
        theme 
    where 
        id_theme_pk = 4))); 


делаю так 

	delete 
		reply
	from 
		reply
	where 
		reply.id_test_fk in ( 
	select 
		id_test_pk 
	from 
		tests
	where 
        tests.id_test_pk in (
	select 
		id_quest_pk 
	from
		quest
	where 
        quest.id_theme_fk = ( 
    select 
        id_theme_pk 
	from 
        theme 
    where 
        id_theme_pk = 4))); 


пишет ошибку 

Error Code: 1175. You are using safe update mode and you tried to update a table without a WHERE that uses a KEY column
 To disable safe mode, toggle the option in Preferences -> SQL Queries and reconnect.	0.000 sec


не смог понять чего это такое. в sql новичек 

SET SQL_SAFE_UPDATES = 0;