Question

I wanted to see if I can better organize / optimize my code, and so I’ve been reading more about joins and how you can query / select from two different tables where a certain column matches up in a single query. However, I could not find any documentation on what I would like to do.

Consider two tables (A , B).

Table A
user_id -- + -- course_id
1       -- + -- 1

Table B
course_id -- + -- project_id
1         -- + --    2

My queries look something along the lines of the following:

$sql_course= mysql_query("SELECT course_id FROM A WHERE user_id = 1") or die(mysql_error());

 while ($course_row = mysql_fetch_assoc($sql_course)) {

                            // Unique course ID
                            $courseID = $course_row['course_id'];

                            $sql_b= mysql_query("SELECT project_id FROM B WHERE course_id=$courseID") or die(mysql_error());

So, you see, this is not very easy to explain. I suppose what I’m looking to find out is whether or not there is a way to optimize this code, say, using one query?

Answer 1

Try:

$sql_course = mysql_query("SELECT A.course_id, B.project_id FROM A INNER JOIN B ON B.course_id = A.course_id WHERE A.user_id = 1 ORDER BY A.course_id, B.project_id") or die(mysql_error());

You only need to include A.course_id in the select portion if you care about the value.