How to loop for every value in table AND find average - php

Question

I am a beginner programmer and I am trying to find the average user rating of vendor 2.
My steps are as follows:
1. User 'Jimmy' gives a rating of 3 to vendor 2 (ratings are out of 5)
2.TABLE vendoratings is updated
3. ratingstot.php then calculates the total sum of ratings and the number of responses for ALL vendors before updating TABLE vendortotalratings shown below
4.User 'Jimmy' clicks on 'View average user rating'
5. The values totalratings and totalno are retrieved and divided in Javascript
var average= totalratings/totalno
6. totalno is displayed to user Jimmy. END
Question
1. I need help forming the for or while loop in ratingstot.php to calculate both ratings and no. of responses belonging to vendor X before inserting them in vendortotalratings table for every vendor.
ratingstot.php
<?php
header("Access-Control-Allow-Origin: *");
header("Content-Type: application/json; charset=UTF-8");
error_reporting(E_ERROR);
try{
//Database connection
$conn = new mysqli("localhost", "XXXXXXXX_XXX", "XXXXXXXX", "XXXXXXXX");
//Unsure how to loop this to make vendor new value every loop
for($i=0; $i<=6; $i++){
$vendor = ??
//Calculate sum of ratings from table ratings
$result = $conn->query("SELECT SUM(ratings) FROM ratings WHERE vendorid = '".$vendorid."' ");
$row = mysqli_fetch_array($result);
$totalratings = $row[0];
//Calculate no. of responses (by counting no. of rows)
$result1 = $conn->query("SELECT * FROM ratings WHERE vendorid = '" . $vendorid."' ");
$totalno = mysqli_num_rows($result);
//inserting the results into the table
$query = " UPDATE vendortotalratings SET ";
$query .= " totalratings = '". $totalratings ."', totalno='".$totalno."' ";
$query .= " WHERE vendorid = '". $vendorid ."'";
$result2 = $conn->query($query);
}
echo($outp);
}
catch(Exception $e) {
$json_out = "[".json_encode(array("result"=>0))."]";
echo $json_out;
}
?>
I have no idea how to loop this, are there any easier steps to calculate average of ratings for each vendors?

Answer 1

Instead of all these complicated things, you can simplify the required solution like this:
(Assumption: I'm assuming that vendorid, ratings, totalratings and totalno columns are of type INT)
Use the below statement/query to get the totalratings and totalno corresponding to each vendorid.
$result = $conn->query("SELECT vendorid, SUM(ratings) as totalratings, COUNT(userid) as totalno FROM vendorratings GROUP BY vendorid");
Now loop through the $result result set using while() loop.
while($row = $result->fetch_array()){
...
}
In each iteration of above while() loop, check if the vendorid value already exists or not. If it exists, then UPDATE the row with new totalratings and totalno, otherwise INSERT a new row comprising of vendorid, totalratings and totalno.
while($row = $result->fetch_array()){
$res = $conn->query("SELECT vendorid FROM vendortotalratings WHERE vendorid = " . $row['vendorid']);
if($res->num_rows){
// Update the existing row
$conn->query("UPDATE vendortotalratings SET totalratings = ".$row['totalratings'].", totalno = ".$row['totalno']." WHERE vendorid = ".$row['vendorid']);
echo "Affected rows: " . $conn->affected_rows . '<br />';
}else{
// Insert a new row
$res = $conn->query("INSERT INTO vendortotalratings VALUES(".$row['vendorid'].", ".$row['totalratings'].",".$row['totalno'].")");
if($res) echo "New row inserted <br />";
}
}
So the complete code of try-catch block would be like this:
// your code
try{
//Database connection
$conn = new mysqli("localhost", "XXXXXXXX_XXX", "XXXXXXXX", "XXXXXXXX");
$result = $conn->query("SELECT vendorid, SUM(ratings) as totalratings, COUNT(userid) as totalno FROM vendorratings GROUP BY vendorid");
while($row = $result->fetch_array()){
$res = $conn->query("SELECT vendorid FROM vendortotalratings WHERE vendorid = " . $row['vendorid']);
if($res->num_rows){
// Update the existing row
$conn->query("UPDATE vendortotalratings SET totalratings = ".$row['totalratings'].", totalno = ".$row['totalno']." WHERE vendorid = ".$row['vendorid']);
echo "Affected rows: " . $conn->affected_rows . '<br />';
}else{
// Insert a new row
$res = $conn->query("INSERT INTO vendortotalratings VALUES(".$row['vendorid'].", ".$row['totalratings'].",".$row['totalno'].")");
if($res) echo "New row inserted <br />";
}
}
}catch(Exception $e) {
$json_out = json_encode(array("result"=>0));
echo $json_out;
}

Answer 2

The question doesn't clarify how the records are first inserted in vendortotalratings table. So, assuming that there is already a record in this table for each vendor, you don't have to write a whole new loop.
Updating vendortotalratings:
SQL can take care of calculating the total ratings and their counts in a single query which can then replace the loop that you have.
UPDATE vendortotalratings vtr
INNER JOIN
(
SELECT vendorid, SUM(ratings) AS sumratings, COUNT(ratings) AS countratings
FROM vendoratings
GROUP BY vendorid
) vr
ON vtr.vendorid = vr.vendorid
SET
vtr.totalratings = vtr.totalratings + vr.sumratings
,vtr.totalno = vtr.totalno + vr.countratings
Computing averages:
As for your second question, to compute the average, you could run the following query which will give you the run-time result:
SELECT vendorid, totalratings, totalno,
CAST((totalratings/totalno) AS DECIMAL(10, 2)) AS avgrating
FROM vendortotalratings;
The variable avgrating can be accessed directly in PHP by using $row['avgrating'] if you're fetching an associative array from the results, or by using the appropriate index number, which in this case should be $row[3]