如何从mysql显示平均值

How to show the average of a column in mysql?

Below is my code which i have tried so far :

<?php
  if (isset($_GET["age"]));
  $age = ($_GET["age"]);
  include($_SERVER["DOCUMENT_ROOT"] . "/includes/config.php");

  // Input
  $sql = "SELECT AVG(column_name) FROM table_name";

  // Check age
  if ($age > 99 or $age < 5) {
    echo ("We only store data of people between the age of 5 and 99.");
    if (!mysqli_query($conn, $sql)) {
      die('Error: ' . ((is_object($GLOBALS["___mysqli_ston"])) ? mysqli_error($GLOBALS["___mysqli_ston"]) : (($___mysqli_res = mysqli_connect_error()) ? $___mysqli_res : false)));
    }
  } 
  else {
    echo ("We got it!");
  }

  // Close connection
  ((is_null($___mysqli_res = mysqli_close($conn))) ? false : $___mysqli_res);

  die();
?>

But how to exactly define a variable to the result of the AVG with a maximum of 2 decimals?

I want to used and show it into another file (so I will include this one).

What I have right now

<?php
if (isset($_GET["age"]));
$age = ($_GET["age"]);
include($_SERVER["DOCUMENT_ROOT"] . "/3/includes/config.php");
include($_SERVER["DOCUMENT_ROOT"] . "/3/includes/opendb.php");

// My own created code
$sql = $conn->query("SELECT ROUND(AVG(price AS FLOAT), 2) FROM data WHERE age= '$age'");
$data = $sql->mysqli_fetch_assoc();
$avg_data = $data['price'];
echo $avg_data;

// This below is from an other post but don't know how it works and if it is good.
$ratings = $conn->query("SELECT AVG(price) avg_rating FROM data form_id = '" . $age . "'");
$data2 = $ratings->mysqli_fetch_assoc();
$avg_rating = $data2['avg_rating'];
echo $avg_rating;

die();

?>

How I fixed it:

<?php

if (isset($_GET["age"])) {
$age = ($_GET["age"]);

include($_SERVER["DOCUMENT_ROOT"] . "/3/includes/config.php");

$con=mysqli_connect($dbhost,$dbuser,$dbpass,$dbname);

if (mysqli_connect_errno($con)) {
    echo "Failed to connect to MySQL: " . mysqli_connect_error();
}

$result = mysqli_query($con,"SELECT AVG(price) FROM data WHERE age= '$age'") or die("Error: " . mysqli_error($con));

while($row = mysqli_fetch_array($result)) {
   echo $row['AVG(price)'];
   echo number_format($row['AVG(price)'], 2);
}

die();
}
else {
echo 'Something went wrong, try again.';
}

?>

Use Like This For Getting Average witth two decimal points.

  $sql = "SELECT ROUND(AVG(column_name AS FLOAT), 2) FROM table_name";

$sql = 'SELECT *, ROUND(AVG(column_name), 2) AS avg_value FROM table_name';

avg_value will store the rounded + average value and add * if need to get all the column.