SQL Query Issues - Sum of client spend per year - php

Question

I'm trying to build what should be (in my mind anyway) a simply SQL query to understand what clients have been invoiced each financial year. The output might look like this:
{clientName (Client A)} - {2010/2011 Value} - {2011/2012 Value} - {2012/2013 Value}
What I've been able to achieve is an output that looks like this:
{clientName (Client A)} - {2010/2011 Value}
{clientName (Client A)} - {2011/2012 Value}
{clientName (Client A)} - {2012/2013 Value}
{clientName (Client B)} - {2010/2011 Value}
And so on…
Now, I know this is not correct but the query I'm working with looks like this:
$query = "SELECT i.invoiceValue, fy.year, c.clientName, c.clientID FROM cms_invoices i
LEFT JOIN cms_financialYear fy ON fy.yearID = i.yearID
LEFT JOIN cms_projects p ON p.projectID = i.projectID
LEFT JOIN cms_clients c ON c.clientID = p.clientID
ORDER BY fy.year, c.clientName";
$result = mysql_query($query) or die(mysql_error());
while($row = mysql_fetch_array($result)) {
echo $row['year'] . " - ";
echo $row['clientName'] . " - $";
echo number_format($row[invoiceValue], 2, '.', ',') . "";
echo "<br>";
I'd be much appreciated if I could get some sort of a steer on this. I've tried for hours but alas, no luck.
Thanks,
#rrfive

Answer 1

You could try something like this:
SELECT c.clientID
, c.clientName
, SUM(IF(fy.year=2010,i.invoiceValue,0)) AS fy_2010
, SUM(IF(fy.year=2011,i.invoiceValue,0)) AS fy_2011
, SUM(IF(fy.year=2012,i.invoiceValue,0)) AS fy_2012
FROM cms_invoices i
LEFT
JOIN cms_financialYear fy ON fy.yearID = i.yearID
LEFT
JOIN cms_projects p ON p.projectID = i.projectID
LEFT
JOIN cms_clients c ON c.clientID = p.clientID
GROUP BY c.clientID, c.clientName
ORDER BY c.clientID, c.clientName
The "trick" is to use an IF function (or a more ANSI portable CASE expression), to determine if a row applies to a given fiscal year. If it does, then return the invoice value, otherwise, return a 0.
Wrap those expressions in a SUM aggregate function, and do the GROUP BY on the client.
If you want to guard against returning a NULL value, then you can wrap those SUM expressions in an IFNULL( ... ,0)