how to count rows in table that has specific property in another table

Question

table1
--------------
| sn | class |
--------------

table2
----------------
| id | student |
---------------- 

all are int as sn is table1 is linked to student in table2 sn , id are auto increasing. when inserting data to table2 student column is same as sn in table 1

now I want to select student in table2 but only those whose class in table1 is "3" my syntax is thus;

$count = mysql_query(SELECT student from table2 whose class in table1 =3)

so that i can count them by saying

$quantity = mysql_num_rows($count)

now my problem is if sql also have this whose keyword, or how do i go about this.

$count = mysql_query(SELECT student from table2 whose class in table1 =3)

Answer 1

You need to join the tables in order to filter the results properly.

(1) This will give you the number of students for class 3.

$count = mysql_query(
  'SELECT COUNT(t2.student) 
   FROM table2 t2 
   INNER JOIN table1 t1
     ON t1.sn = t2.student
     AND t1.class = 3'
);

(2) This will give you all classes and the number of students for each.

$count = mysql_query(
  'SELECT t1.class, COUNT(t2.student) 
   FROM table2 t2 
   INNER JOIN table1 t1
     ON t1.sn = t2.student
   GROUP BY t1.class
   ORDER BY t1.class'
);

(3) This will give you all classes and the students list.

$list = mysql_query(
  'SELECT t1.class, GROUP_CONCAT(t2.student SEPARATOR ',') 
   FROM table2 t2 
   INNER JOIN table1 t1
     ON t1.sn = t2.student
   GROUP BY t1.class
   ORDER BY t1.class'
);

Answer 2

You should join those two tables and limit your result to those which have table1.class = 3

SELECT
    student
FROM
    table2 a
    JOIN table1 b ON (a.student = b.sn)
WHERE b.class = 3

If you want a count you could also do it through SQL by using aggregate function

SELECT
    COUNT(student)
FROM
    table2 a
    JOIN table1 b ON (a.student = b.sn)
WHERE b.class = 3