MySQL Sub Sub Query seems to loop - mysql

Question

I have this MySQL query to get the total amount of only the first invoice for each client on a given month:
SELECT SUM(InvoiceProductTotal)
FROM tblinvoiceproduct
WHERE InvoiceID IN (
SELECT MIN(tblinvoice.InvoiceID) AS InvoiceID
FROM tblinvoice
WHERE tblinvoice.ClientID IN (
SELECT tblclient.ClientID
FROM tblclient
LEFT JOIN tblenquiry ON tblclient.EnquiryID = tblenquiry.EnquiryID
WHERE NOT tblclient.EnquiryID IS NULL
AND YEAR(EnquiryDate) = 2014
AND MONTH(EnquiryDate) = 9
)
GROUP BY tblinvoice.ClientID
);
When I run it, it seems to loop forever. If I remove the first part it gives me the list of invoices instantly. Am sure it is a small syntax detail but haven't been able to figure out what the problem is after nearly one hour trying to fix it.
Your assistance is appreciated.
This query can probably be done in a better way without all the sub queries as well, just I'm not so experienced with sub queries. :)
Solution was given but I should have included the full query rather than just the part I was having trouble with. The full query is:
SELECT AdvertisingID, AdvertisingTitle, AdvertisingYear,
AdvertisingMonth, AdvertisingTotal, AdvertisingVisitors,
IFNULL(
(SELECT SUM(InvoiceProductTotal)
FROM tblinvoiceproduct
JOIN
(SELECT MIN(tblinvoice.InvoiceID) AS InvoiceID
FROM tblinvoice
JOIN
(SELECT DISTINCT tblclient.ClientID
FROM tblclient
JOIN tblenquiry ON tblclient.EnquiryID = tblenquiry.EnquiryID
WHERE YEAR(tblenquiry.EnquiryDate)=tbladvertising.AdvertisingYear
AND MONTH(tblenquiry.EnquiryDate)=tbladvertising.AdvertisingMonth)
AS inq
ON tblinvoice.ClientID = inq.ClientID
GROUP BY tblinvoice.ClientID) AS inq2
ON tblinvoiceproduct.InvoiceID = inq2.InvoiceID)
, 0)
FROM tbladvertising
ORDER BY AdvertisingYear DESC, AdvertisingMonth DESC, AdvertisingTitle;
Now the problem is that the column with the sub query has no access to "tbladvertising.AdvertisingYear" or "tbladvertising.AdvertisingMonth"

Answer 1

A commenter mentioned that it's hard to understand what you're trying to do here. I agree. But I will take the risk of trying to puzzle it out.
As usual with this sort of query, it's helpful to take advantage of the structured part of structured query language, and try to build this up piece by piece. That's the secret to creating complex queries that actually do what you want them to do.
Your innermost query is this:
SELECT tblclient.ClientID
FROM tblclient
LEFT JOIN tblenquiry ON tblclient.EnquiryID = tblenquiry.EnquiryID
WHERE NOT tblclient.EnquiryID IS NULL
AND YEAR(EnquiryDate) = 2014
AND MONTH(EnquiryDate) = 9
It is saying, "give me the list of ClientID values which have enquiries in September 2014. There's a more efficient way to do this:
SELECT DISTINCT tblclient.ClientID
FROM tblclient
JOIN tblenquiry ON tblclient.EnquiryID = tblenquiry.EnquiryID
WHERE tblenquiry.EnquiryDate >= '2014-09-01'
AND tblenquiry.EnquiryDate < '2014-09-01' + INTERVAL 1 MONTH
Two changes here: First, the NOT ... IS NULL search is unnecessary because if the item you're searching on is null, there's no way for your EnquiryDate to be valid. So we just change the LEFT JOIN to an ordinary inner JOIN and get rid of the otherwise expensive NULL scan.
Second, we recast the date matching as a range scan, so it can use an index on tbl.EnquiryDate.
Cool.
Next, we have this query level.
SELECT MIN(tblinvoice.InvoiceID) AS InvoiceID
FROM tblinvoice
WHERE tblinvoice.ClientID IN (
/* that list of Client IDs from the innermost query */
)
GROUP BY tblinvoice.ClientID
That is pretty straightforward. But MySQL isn't too swift with IN () clauses, so let's recast it in the form of a JOIN as follows:
SELECT MIN(tblinvoice.InvoiceID) AS InvoiceID
FROM tblinvoice
JOIN (
/* that list of Client IDs from the innermost query */
) AS inq ON tblinvoice.ClientID = inq.ClientID
GROUP BY tblinvoice.ClientID
This gets us the list of invoice IDs which were the subject of the first enquiry of the month on behalf of each distinct ClientID. (It's hard for me to figure out the business meaning of this, but I don't understand your business.)
Finally, we come to your outermost query. We can also recast that as a JOIN, like so.
SELECT SUM(InvoiceProductTotal)
FROM tblinvoiceproduct
JOIN (
/* that list of first-in-month invoices */
) AS inq2 ON tblinvoiceproduct.InvoiceID = inq2.InvoiceID
So, this all expands to:
SELECT SUM(InvoiceProductTotal)
FROM tblinvoiceproduct
JOIN (
SELECT MIN(tblinvoice.InvoiceID) AS InvoiceID
FROM tblinvoice
JOIN (
SELECT DISTINCT tblclient.ClientID
FROM tblclient
JOIN tblenquiry ON tblclient.EnquiryID = tblenquiry.EnquiryID
WHERE tblenquiry.EnquiryDate >= '2014-09-01'
AND tblenquiry.EnquiryDate < '2014-09-01' + INTERVAL 1 MONTH
) AS inq ON tblinvoice.ClientID = inq.ClientID
GROUP BY tblinvoice.ClientID
) AS inq2 ON tblinvoiceproduct.InvoiceID = inq2.InvoiceID
That should do the trick for you. In summary, the big optimizing changes are
using a date range scan.
eliminating the NOT ... IS NULL criterion.
recasting your IN clauses as JOIN clauses.
The next step will be to create useful indexes. A compound index (EnquiryDate, EnquiryID) on your tblenquiry is very likely to help a lot. But to be sure you'll need to do some EXPLAIN analysis.

Answer 2

What if you modify your above posted query, to replace the subquery with JOIN (INNER JOIN) like below. Give it a try.
SELECT SUM(InvoiceProductTotal)
FROM tblinvoiceproduct
JOIN
(
SELECT MIN(ti.InvoiceID) as MinInvoice
FROM tblinvoice ti
JOIN
(
SELECT tblclient.ClientID
FROM tblclient
LEFT JOIN tblenquiry
ON tblclient.EnquiryID = tblenquiry.EnquiryID
WHERE NOT tblclient.EnquiryID IS NULL
AND YEAR(EnquiryDate) = 2014
AND MONTH(EnquiryDate) = 9
) tab
on ti.ClientID = tab.ClientID
GROUP BY ti.ClientID
) tab1
on tblinvoiceproduct.InvoiceID = tab1.MinInvoice