从一个表中添加多个字段的值，并将其减去到另一个表中的字段中的PHP

Question

I have two tables : contract bill .
contract table contains following fields(contId (PK), contractNumber, quantity, etc..).
bill contains following fields(billId (PK), billNo, contId, checkMtr, etc...).

Here every contract can have multiple bills. What I am trying to do is add all the inwMtr of a contract and deduct that from the quantity and display the final mtrs as Pending Mtr.

Here is what I have tried:

<?php

include("dbconnection.php");

$sql1 = mysql_query("SELECT quantity FROM contract WHERE contId = '25'");
while($result1  = mysql_fetch_array($sql1))
{
//echo "$result1[quantity]";
//echo "<br/>";


//echo "<br/>";

$sql2 = mysql_query("SELECT checkMtr FROM bill WHERE contId = '25'");
while($result2  = mysql_fetch_array($sql2))
{
//echo "$result2[checkMtr]";
//echo "<br/>";

$a = $result1['quantity'] - $result2['checkMtr'];
echo "$a";
echo "<br/>";

}
echo "<br/>";

}
echo "<br/>";
?>

Here it does subtract checkMtr from quantity but it does it for every bill. It doesn't add checkmtrs of all the bills and then subtract.

Any suggestions?

Answer 1

$sql2 = mysql_query("SELECT sum(checkMtr) checkMtr FROM bill WHERE contId = '25'");
if (mysql_num_rows($sql2) == 1) {
  $result2  = mysql_fetch_array($sql2);
  $a = $result1['quantity'] - $result2['checkMtr'];
} else {
  $a = $result1['quantity'];
}
echo $a;

There's also no need for the while loop after the first query. It can never return more than one row, since you're selecting the row by primary key.

You can also combine this into one query:

SELECT quantity - ifnull(sum(checkMtr), 0) result
FROM contract c
LEFT JOIN bill USING (contId)
WHERE c.contId = '25'

To do this for all contracts, use GROUP BY :

SELECT c.contId, quantity - ifnull(sum(checkMtr), 0) result
FROM contract c
LEFT JOIN bill USING (contId)
GROUP BY c.contId

SQLFIDDLE

Answer 2

Replace checkMtr both in the SQL statement and in your subtraction line with

SUM(checkMtr)

and it will work.

Answer 3

If I understand you correctly you want checkMtr to be a single number that is subtracted?

This will add all of the checkMtr results and will output that sum.

SELECT sum(checkMtr) FROM bill WHERE contId = '25'

Also if contId is being submitted from the user I hope you are validating the data