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