A page needs to query the contents of multiple tables, how to do?

A page needs to query the contents of multiple tables, how to operate??
Course Table
Idteacheridclassid Nameroomremarks
Teacher Number Class Number Course name Classroom notes


Student table
Idnamepasswordclassid
School Number name Code class


Known student ID, you need to check the student table to get the corresponding ClassID, and then through the course to find the corresponding class of course information output


The Written statement is:
$result = mysql_query ("SELECT * from Course". " WHERE class= "." Select ClassID from student "." Where id= '. $username. "'". ".", $conn);

Want to write in two statements, but do not know how to save the intermediate results.

However, when opened, the following error message appears:


Ask for advice!


------Solution--------------------
$result = mysql_query ("SELECT * from Course where class= (select ClassID from student where id= '". $username. "')", $conn);

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