Combine multiple queries to get single sum answer - php

Question

Quite new to all the SQL/PHP stuff - dabbled with basic queries and outputting them to PHP previously but now trying something a bit more complicated and hoping someone can help with this as I've been trying to work it out with no luck so far:
I have 2 MS SQL tables:
Table 1 - Faults
faultid ... requestnumber
1 ........... 6
2 ........... 5
3 ........... 6
Table 2 - actions
faultid ....who ..... when...... timetaken
1.......... John....... Mon......... 1.00
2.......... Peter...... Mon.......... 2.00
3.......... Luke....... Tues........ 1.00
2.......... John....... Tues........ 0.5
1.......... Mike....... Mon......... 0.75
What I am trying to achieve is create a variable I can use in a front end php based webpage that gets a sum of the timetaken column in Table 2 where the requestnumber in Table 1 is equal to a specific number (i.e. 6)
I'm guessing it will start with something like:
$sql1 = "select faultid FROM Faults WHERE requestnumber = '6'";
$sqlresult = sqlsrv_query($conn, $sql1);
while ($row = sqlsrv_fetch_array($sqlresult)){
}
After that I get a bit stuck. How do I take each result from this and then run another query to get the sum of the timetaken column in Table 2 for just the corresponding faultid's? I want to hazard a guess at using foreach but not sure on the syntax (or even if I'm guessing correctly).
So in this example I would get back a result of 2.75 as a variable in PHP.
Any help would be appreciated. Thanks in advance.
Andy

Answer 1

Best to use just one SQL-statement
$sql = 'SELECT SUM(t2.timetaken)';
$sql .= ' FROM Faults t1 INNER JOIN actions t2 ON (t2.faultid = t1.faultid)';
$sql .= ' WHERE t1.requestnumber = ?';
Use this as prepared statement and pass your requestnumber (6 or something) as argument when executing this statement.
To get all or multiple sums you can use group by (maybe combine with WHERE):
SELECT t1.requestnumber, SUM(t2.timetaken)
FROM Faults t1
INNER JOIN actions t2 ON (t2.faultid = t1.faultid)
GROUP BY t1.requestnumber
Edit:
According to your own comment, you can use a SELECT with subquery, but use IN, not =. When using '=' the subquery must return only one row.
select SUM (timetaken) FROM actions WHERE faultid IN (select Faultid from Faults WHERE requestnumber = '6')
-- ^^
But this way is usually slower than the one I posted above