如何将这个嵌套的php / mysql查询更改为1个更高效的查询？

Question

I'm looking for some help making my php/MySQL code more efficient. At the moment I've got a nested SQL statement in my PHP code which is taking forever to run. I know there is a more efficient way and doing the query in one statement but I'm struggling work out how to do it.

Basically, I have 2 tables. 'customers' and 'purchases'. I want to run through the 'customers' table and count/display how many purchases they have made from the 'purchases' table.

This is my php/mysql code:

$sql = "SELECT CustomerID, Username, Active FROM customers WHERE AND Active = 'Y'"; 
$result = mysql_query($sql,$connection);
while ($myrow = mysql_fetch_array($result)) {

    $sql2 = "SELECT COUNT(ID) FROM purchases WHERE AND (CustomerID = $myrow[CustomerID] AND (Date BETWEEN $date1 AND $date2) )
    $result2 = mysql_query($sql2,$connection);
    $TotalPurchases = mysql_result($result2,0,”count(ID)”);

}

Which outputs:

MrSmith: 10
MrsGreen: 4
MrGrey: 1
MissDonna: 0

I could probably turn it into 1 query if it wasn't for the variables $date1 and $date2 (Date BETWEEN $date1 AND $date2) which is calculated in php. Any advice on how to make this a more efficient query?

Thanks

Carlos

Answer 1

Please try this query,

SELECT c.CustomerID, Username, Active , COUNT(p.CustomerID)
    FROM customers as c
    left join purchases as p ON c.CustomerID = p.CustomerID
         AND Date BETWEEN $date1 AND $date2
    group by p.CustomerID 

I think this can help you.

Answer 2

SELECT  c.CustomerID, c.Username, c.Active , COUNT(*)
    FROM  customers as c
    join  purchases as p ON c.CustomerID = p.CustomerID
      AND  p.Date BETWEEN $date1 AND $date2
    group by  p.CustomerID 

Notes: JOIN, not LEFT JOIN. COUNT(*).

purchases should have compound INDEX(Date, CustomerID) .

This might be even faster ( Edited ):

SELECT  c.CustomerID, c.Username, c.Active , p.ct
    FROM ( SELECT CustomerID, COUNT(*) ct FROM purchases
              WHERE Date BETWEEN $date1 AND $date2
              GROUP BY CustomerID ) p
    JOIN  customers AS c   ON c.CustomerID = p.CustomerID