An error occurred while updating mysql table content in java.

SQL statement: publicstaticvoiddeleteUser (StringloginName) {Stringsql & quot; updateusersetflag3whereloginName & quot; + loginName; DBUtil. getInstance (). delete (SQL);} error message: java. SQL. SQLException: Datatruncation: TruncatedincorrectDOUBLEvalue: mysqljava

SQL statement:
Public static void deleteUser (String loginName ){
String SQL = "update user set flag = 3 where loginName =" + loginName;
DBUtil. getInstance (). delete (SQL );
}
Error message:
Java. SQL. SQLException: Data truncation: Truncated incorrect DOUBLE value: 'admin' Query: update user set flag = 3 where loginName = 123654 Parameters: []
Why is it wrong?