Resourceid #6

Resourceid #6 test page error:
Error page:/index. php
Error infos: No database selected
Error SQL: select * from c_seo where id = 1

I don't know where the problem is. Can someone help me?
$ This-> The queryString value is select * from c_seo where id = 1
$ This-> the value of linkID is Resource id #6.

 Execute ($ id, $ SQL);} function Execute ($ id = "me", $ SQL = '') {global $ DB; if ($ DB-> isClose) {$ this-> Open (false); $ DB-> isClose = false;} if (! Empty ($ SQL) {$ this-> SetQuery ($ SQL);} // if ($ this-> safeCheck) {CheckSql ($ this-> queryString) ;}$ this-> result [$ id] = mysql_query ($ this-> queryString, $ this-> linkID ); if ($ this-> result [$ id] === false) {$ this-> DisplayError (mysql_error ()."
Error SQL :". $ this-> queryString. "") ;}} function DisplayError ($ msg) {$ errorTrackFile = dirname (_ FILE __). '/.. /data/mysql_error_trace.inc '; if (file_exists (dirname (_ FILE __). '/.. /data/mysql_error_trace.php ') {@ unlink (dirname (_ FILE __). '/.. /data/mysql_error_trace.php ');} $ emsg = ''; $ emsg. ="
Error Warning! \ R \ n "; $ emsg. ="
\ R \ n "; $ emsg. ="

Error page: ". $ this-> GetCurUrl ()."
\ R \ n "; $ emsg. ="
Error infos: {$ msg}
\ R \ n "; $ emsg. ="
\ R \ n "; echo $ emsg;}?>

Reply to discussion (solution)

No database selected
Check ......

Error infos: No database selected: No database is selected.

Configuration error!

No database selected
Check ......

Embarrassed... I checked it again and found that the database name was wrong... the upper and lower bars were wrong.
Thank you