如何使用按日期分组的SUM（）和MAX（）连接2个表

Question

i have table loadhistory (planning to "select only highest value per date" group by and order by date DESC)

| user_id | customer_id |     date    | bal |
    1           1         2015-02-27    500
    2           1         2015-02-27    650
    3           1         2015-02-28    450
    4           1         2015-02-28    620

and table transactionrecord (and planning to sum up values per date using SUM(bal) group by and order by date DESC)

| user_id | customer_id |     date     | bal |
    1           1         2015-02-27     50
    2           1         2015-02-27     20
    3           1         2015-02-28     10

But i want to join the 2 tables which would look like this:

|    date    |   balance    |   amount paid   |
  2015-02-28       620               10
  2015-02-27       650               70

im not good in joining tables. this is my code so far and not working

$q = "SELECT a.customer_id, SUM(a.bal), a.date, MAX(b.bal) GROUP BY date 
FROM transactionrecord as a 
LEFT JOIN loadhistory as b ON b.customer_id = a.customer_id
WHERE customer_id = {$_COOKIE['id']} GROUP BY date 
ORDER BY date DESC";
$r = @mysqli_query ($dbc, $q );

echo '<table align="center" cellspacing="0" cellpadding="5" width="45%">
<tr>
<td align="center"><b>Date</b></td>
<td align="center"><b>Balance</b></td>
<td align="center"><b>>Amount Paid></b></td>
</tr>';
    while ($row = mysqli_fetch_array($r, MYSQLI_ASSOC)) {
    echo '
    <td align="center">' . $row['date'] . '</td>
    <td align="center">' . $row['MAX(b.bal)'] . '</td>
    <td align="center">' . $row['SUM(a.bal)'] . '</td>

';  

what to change in my query to join the 2 tables with the SUM() and MAX() included? And is my using of $row[' '] in echo right?

thanks a lot.

Answer 1

Your SQL statement should look like this in order to achieve your desired result:

SELECT a.customer_id, a.date, MAX(COALESCE(b.bal, 0)) AS bal, a.paid
FROM (
  SELECT customer_id, date, SUM(bal) AS paid
  FROM transactionrecord
  GROUP BY customer_id, date
) AS a LEFT JOIN loadhistory AS b
  ON a.customer_id = b.customer_id AND a.date = b.date
WHERE a.customer_id = 1
GROUP BY a.customer_id, a.date, a.paid
ORDER BY a.date DESC

And in your php, you cannot refer to result columns using MAX(b.bal) or SUM(a.bal) ; instead, you must alias the columns as I've done above. So you can refer to MAX(b.bal) as bal and you can refer to SUM(a.bal) as paid .

You mostly had the SQL right, I only

• removed a misplaced GROUP BY expression,
• added aliases for the aggregated ( SUM and MAX ) columns,
• put the transaction balance summing in a subquery to prevent multiplicative results for multiple rows in the loadhistory table
• qualified all mentions of date and customer_id columns, since those columns appear in both tables,
• added a COALESCE on bal in case the left join results in no matching loadhistory records,
• added a join criterion on date since records must have equivalent customer_id and date to match your requirements, and
• added customer_id to the GROUP BY clause because any non-aggregated fields in the SELECT clause should be in the GROUP BY clause for predictable results.

If you wanted to select the value with highest user_id per date in the table loadhistory instead of MAX(bal) , you would need to do something like this:

SELECT b.user_id, a.customer_id, a.date,
  COALESCE(b.bal, 0) AS bal, SUM(a.bal) AS paid
FROM transactionrecord AS a LEFT JOIN (
  SELECT h1.user_id, h1.customer_id, h1.date, h1.bal
  FROM loadhistory h1 INNER JOIN (
    SELECT MAX(user_id) AS user_id, customer_id, date
    FROM loadhistory GROUP BY customer_id, date
  ) AS h2 ON h1.user_id = h2.user_id
         AND h1.customer_id = h2.customer_id
         AND h1.date = h2.date
) AS b ON a.customer_id = b.customer_id AND a.date = b.date
WHERE a.customer_id = 1
GROUP BY b.user_id, a.customer_id, a.date, b.bal
ORDER BY a.date DESC