PHP用数组降低程序的时间复杂度

				
$querystr = "select distinct S.STUNAME as STUNAME,C.CLASSNAME as CLASSNAME ".
   "from STUDENTS as S,CLASSES as C,SCORES as R ".
   "where S.SID=R.SID and S.CLASSID=C.CLASSID and R.COURSE='Math' ".
			"and R.SCORE>=90";		
$result = $db2handle->query( $querystr ); //从数据库中获取数据
while( $row=$result->fetchRow(DB_FETCHMODE_ASSOC) ){
 //读取并显示数据
 echo "StudentName=".$row['STUNAME']."\t ClassName=".$row['CLASSNAME']."\n"; 
}//Done

				
$scorestr = "select distinct SID from SCORES where COURSE='Math' and SCORE>=90";
$scoredata = $db2handle->query( $scorestr ); 
//从数据库中获取满足条件的学生学号
while( $score=$scoredata->fetchRow(DB_FETCHMODE_ASSOC) ){
 //读取学生的学号，并在STUDENTS表中查找学生的姓名和班级编号
 $studentstr = "select STUNAME,CLASSID from STUDENTS where SID='".$score['SID']."'";
 $studata =$db2handle->query( $studentstr);
 $stu=$studata->fetchRow(DB_FETCHMODE_ASSOC);
 //显示学生的姓名
 echo "StudentName=".$stu['STUNAME']."\t ";
 //读去学生的班级编号，并在CLASSES表中查找该学生所在班级名称
 $classstr = "select CLASSNAME from CLASSES where CLASSID='".$stu['CLASSID']."'";
 $classdata = $db2handle->query( $classstr);
 $class=$classdata ->fetchRow(DB_FETCHMODE_ASSOC);
 //显示学生的班级
 echo "CLASSNAME=".$class['CLASSNAME']."\n";
}//end while for getting each student's ID. Done

				
$ClassArray = Array();
$StuArray = Array();
$classstr = "select CLASSID,CLASSNAME from CLASSES";
$classdata = $db2handle->query( $classstr);
while( $class=$classdata ->fetchRow(DB_FETCHMODE_ASSOC) ){
 //生成ClassArray数组，下标Index以CLASSID命名，对应的值为CLASSNAME
 $ClassArray[$class['CLASSID']] = $class['CLASSNAME'];
}//end while $ClassArray
$stustr="select SID,STUNAME,CLASSID from STUDENTS";
$studata = $db2handle->query( $stustr);
while( $stu=$studata ->fetchRow(DB_FETCHMODE_ASSOC) ){
 //生成StuArray数组，下标Index以SID命名，对应的值为STUNAME和CLASSID
 $StuArray[$stu ['SID']]['STUNAME'] = $stu['STUNAME'];
 $StuArray[$stu ['SID']]['CLASSID'] = $stu['CLASSID'];
}//end while $StuArray
$scorestr = "select distinct SID from SCORES where COURSE='Math' and SCORE>=90";
$scoredata = $db2handle->query( $scorestr ); 
//从数据库中获取满足条件的学生学号
while( $score=$scoredata->fetchRow(DB_FETCHMODE_ASSOC) ){
 //读取学生的学号，并从StuArray中读取学生的姓名，从ClassArray中读取班级名称
 echo "StudentName=".$StuArray[ $score['SID'] ]['STUNAME']."\t ";
 echo "CLASSNAME=".$ClassArray[ $StuArray[ $score['SID'] ]['CLASSID'] ]."\n";
}//end while for getting each student's ID. Done

				
$ClassArray = Array();
$classstr = "select CLASSID,CLASSNAME from CLASSES";
$classdata = $db2handle->query( $classstr);
while( $class=$classdata ->fetchRow(DB_FETCHMODE_ASSOC) ){
 //生成ClassArray数组，下标Index以CLASSID命名，对应的值为CLASSNAME
 $ClassArray[$class['CLASSID']] = $class['CLASSNAME'];
}//end while $ClassArray
$stustr = "select STUNAME,CLASSID from STUDENTS where SID in ".
   "(select distinct SID from SCORES where COURSE='M' and SCORE>=90)";
$studata = $db2handle->query( $stustr); 
//从数据库中获取满足条件的学生姓名和班级编号
while( $stu=$studata ->fetchRow(DB_FETCHMODE_ASSOC) ){
 //读取学生的姓名，并从ClassArray中读取班级名称
 echo "StudentName=".$stu ['STUNAME']."\t ";
 echo "CLASSNAME=".$ClassArray[ $stu ['CLASSID'] ]."\n";
}//end while for getting each student's Info. Done