如何从mysql中的两个不同表中选择两列[duplicate]

Question

Possible Duplicate:
how to select two columns from two tables in mysql

I am trying to execute this query but i got different output. I want to count row from one column(grade) from table grading and select column(level) from other table info.

for($i=1; $i<9; $i++)
{
$result = mysql_query("SELECT COUNT(grade),level FROM grading,info WHERE grade = $i");
 while ($row = mysql_fetch_array($result))
 {

    echo "<tr><td>";
    echo $row['level'];
    echo "</td>";
    echo "<td>";
    echo  $row['COUNT(grade)'];
    echo "</td></tr>";
 }
}

my database:

      level   grade          

        A       1
        B       2
        C       6
        D       3
        E       4
        F       5
        G       6
        H       8

My output:

      level   grade     
        A       8
        B       8
        C       8
        D       8
        E       8
        F       16
        G       0
        H       8

but its wrong i want to count how many students have grade 1,2,3,4,5,6 but output display something else.

Answer 1

I think, you must use JOIN http://en.wikipedia.org/wiki/Join_(SQL) instead of FROM table1, table2.