Sum fields from different tables per userID - php

Question

Need your help.
php, mysql
I have the following project.
I have two tables
**table 1**
user_id Plan
1 5
1 7
2 5
2 9
3 7
1 9
**table 2**
Plan Price
5 100
7 200
9 300
I must find the total cost of plans selected by one user
eg user_id = 2 must pay 400
I have already the following code, but this one adds the Price of all Plans in database in the above example total cost = 600
What am I doing wrong? :(
$totalcost = NULL;
$sql = "select SUM(Plan.Cost) as ANSWER FROM Plan";
$result = mysql_query($sql, $link) or die(mysql_error());
$totalcost = mysql_fetch_row($result);
$sql = "select * FROM Plan";
$result = mysql_query($sql, $link) or die(mysql_error());
$rows = mysql_num_rows($result);
$Plan = array();
if (is_resource($result)) {
while ($row = mysql_fetch_array($result, MYSQL_ASSOC)) {
$Plan[] = $row;
}
}

Answer 1

You have to specify the user_id in your query, like:
$user_id = 2; // or get it from $_GET, $_POST...
$sql = "select SUM(Plan.Cost) as ANSWER
FROM Plan, Users
WHERE Users.Plan = Plan.Plan AND Users.user_id = $user_id";

Answer 2

You probably want to use LEFT JOIN in your query, so you can make something like this:
SELECT table1.user_id, table1.plan, SUM(table2.cost) FROM table1 LEFT JOIN table2 ON table1.plan=table2.plan WHERE table1.user_id = $user_id;
this way you can fetch results in 1 query and make database do all the work instead of looping through data in functions etc.
SQL left join tutorial