set @name="This is a very long string, that mustn't find room in a system field like Table_name. Thus it should be cut by the actual size of the field. So we can use this string to find out the actual length of the field and to use it in any compare queries"; create table test_db select * from mysql.db; delete from test_db; insert ignore into test_db (Host,Db,User) values (@name,@name,@name); create table test_host select * from mysql.host; delete from test_host; insert ignore into test_host (Host,Db) values (@name,@name); create table test_user select * from mysql.user; delete from test_user; insert ignore into test_user (Host,User) values (@name,@name); create table test_func select * from mysql.func; delete from test_func; insert ignore into test_func (name) values (@name); create table test_tables_priv select * from mysql.tables_priv; delete from test_tables_priv; insert ignore into test_tables_priv (Host,Db,User,Table_name) values (@name,@name,@name,@name); create table test_columns_priv select * from mysql.columns_priv; delete from test_columns_priv; insert ignore into test_columns_priv (Host,Db,User,Table_name,Column_name) values (@name,@name,@name,@name,@name); select if(isnull(test_db.Host),'WRONG!!!','ok') as test_db_Host, if(isnull(test_host.Host),'WRONG!!!','ok') as test_host_Host, if(isnull(test_user.Host),'WRONG!!!','ok') as test_user_Host, if(isnull(test_tables_priv.Host),'WRONG!!!','ok') as test_tables_priv_Host, if(isnull(test_columns_priv.Host),'WRONG!!!','ok') as test_columns_priv_Host from test_db left join test_host on test_db.Host=test_host.Host left join test_user on test_db.Host=test_user.Host left join test_tables_priv on test_db.Host=test_tables_priv.Host left join test_columns_priv on test_db.Host=test_columns_priv.Host; test_db_Host test_host_Host test_user_Host test_tables_priv_Host test_columns_priv_Host ok ok ok ok ok select if(isnull(test_db.Db),'WRONG!!!','ok') as test_db_Db, if(isnull(test_host.Db),'WRONG!!!','ok') as test_host_Db, if(isnull(test_tables_priv.Db),'WRONG!!!','ok') as test_tables_priv_Db, if(isnull(test_columns_priv.Db),'WRONG!!!','ok') as est_columns_priv_Db from test_db left join test_host on test_db.Db=test_host.Db left join test_tables_priv on test_db.Db=test_tables_priv.Db left join test_columns_priv on test_db.Db=test_columns_priv.Db; test_db_Db test_host_Db test_tables_priv_Db est_columns_priv_Db ok ok ok ok select if(isnull(test_db.User),'WRONG!!!','ok') as test_db_User, if(isnull(test_user.User),'WRONG!!!','ok') as test_user_User, if(isnull(test_tables_priv.User),'WRONG!!!','ok') as test_tables_priv_User, if(isnull(test_columns_priv.User),'WRONG!!!','ok') as test_columns_priv_User from test_db left join test_user on test_db.User=test_user.User left join test_tables_priv on test_db.User=test_tables_priv.User left join test_columns_priv on test_db.User=test_columns_priv.User; test_db_User test_user_User test_tables_priv_User test_columns_priv_User ok ok ok ok select if(isnull(test_tables_priv.User),'WRONG!!!','ok') as test_tables_priv_User, if(isnull(test_columns_priv.User),'WRONG!!!','ok') as test_columns_priv_User from test_tables_priv left join test_columns_priv on test_tables_priv.Table_name=test_columns_priv.Table_name; test_tables_priv_User test_columns_priv_User ok ok drop table test_columns_priv; drop table test_tables_priv; drop table test_func; drop table test_host; drop table test_user; drop table test_db;