How Do I Keep Adding To This Array? - php

Question

I am running the following code, which runs a query, gets all the student_id's in a class, then for each student id it gets all their grades. My problem is though that the data put into the array doesnt stay in it when it goes back and runs the for loop for the second, third, etc student. Is there a way to keep the data in the array and keep adding to it? I have included some code below to help paint the picture. Thanks All
$sql = "SELECT student_id FROM users_table WHERE class_id ='5'";
$result = $db->sql_query($sql);
$row = $db->sql_fetchrow($result);
$row_cnt = mysqli_num_rows($result);
$students = array();
for($y=0; $y<$row_cnt; $y++)
{
array_push($students, $row['student_id']);
$sql2 = "SELECT student_grade FROM grades_table WHERE student_id ='".$students[$y]."'";
$result2 = $db->sql_query($sql2);
$row2 = $db->sql_fetchrow($result2);
$row_cnt2 = mysqli_num_rows($result2);
for($z=0; $z<$row_cnt2; $z++)
{
array_push($students, array($row['student_grade']));
}
}

Answer 1

In the second sql2 part you may want to do something like this instead:
for($z=0;$z<$row_cnt2;$z++) {
$students[$row['student_id']] = $row['student_grade'];
}
Doing it like this, your array doesn't get overwritten, and you'll get something like this:
array(
5 => "A"
6 => "B"
)
Where 5 is the student_id and A is the grade.
But, of course, this can also easily be solved with the JOIN query Mark Baker provided.
What's the "larger query" you're writing?

Answer 2

What you are doing at the moment will make the students array look like a big basket with everything poured in... You should create an index then assign the grades array to it
$students = array();
for($i = 0; $i < $row_cnt; $i++)
{
$sql2 = "SELECT student_grade FROM grades_table WHERE student_id ='".$students[$y]."'";
$result2 = $db->sql_query($sql2);
$row2 = $db->sql_fetchrow($result2);
$row_cnt2 = mysqli_num_rows($result2); //probably don't need this count anymore
$students[$row['student_id']] = $row2;
}
You should take note of Mark's comment... it provides a more elegant way. Also learn about prepared statements and how the help prevent SQL injection