shortcodes, custom php and their errors

UPDATE- After some suggestions I changed this over to using the $wpdb API. The following code is retuning a ‘1’ instead of the actual result count which should be ‘2’.

//a function is a block of code we can call everything between { } is the functions code. The return is the output.
function reservations_yesterday() {
global $wpdb;

              //query DB… get_var should get the output from the db and store it in the results variable. 
$results = $wpdb->get_var ("SELECT SUM(`mattiscool`), booking_date FROM `wp_cbxrbooking_log_manager` WHERE `booking_date` = CURDATE() -1");

//return the results, sends the output back to the code that calls the function.
return $results;
}


// this is a function, and you that calls the reservations_yesterday functions, and you are assigning the shortcode tag of res-1
add_shortcode('res-1', 'reservations_yesterday');

Original – Can someone please lead a man to learn how to fish on this one? I have a customshort codes file. I know that the file and its hook for the first code is right since it functions. I am trying to add a shortcode with an SQL query with a SUM. I know that the sql string is good since it has been validated. I am getting errors on degudding that I dont get. Previously I was using the same php code with a plug in but am now moving to shortcodes and not using the plugin.

code:

// how many reservations yesterday
function res_yesterday(){

$servername = "localhost";
$username = "site";
$password = "pass";
$dbname = "site";

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($conn->connect_error) {
  die("Connection failed: " . $conn->connect_error);
}

$sql = "SELECT SUM(`mattiscool`) ,booking_date as 'total' FROM `wp_cbxrbooking_log_manager` WHERE `booking_date` = CURDATE() -1";

$result = mysqli_query($sql);

while ($row = mysqli_fetch_assoc($result))
{ 
   echo $row['total'];
}

mysqli_close($con);


}
add_shortcode('res_1', 'res_yesterday');

errors:

Warning: mysqli_query() expects at least 2 parameters, 1 given in /home/food/domains/xxx.com/public_html/wp-content/themes/yummy/custom-shortcodes.php on line 29

Warning: mysqli_fetch_assoc() expects parameter 1 to be mysqli_result, null given in /home/food/domains/xxx.com/public_html/wp-content/themes/yummy/custom-shortcodes.php on line 31

Warning: mysqli_close() expects parameter 1 to be mysqli, null given in /home/food/domains/xxx.com/public_html/wp-content/themes/yummy/custom-shortcodes.php on line 36

Can someone please tell me what I am doing wrong?

Answers:

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Method 1

The answer is:

function reservations_yesterday() {
global $wpdb;


$results = $wpdb->get_var ("SELECT SUM(`mattiscool`), booking_date FROM `wp_cbxrbooking_log_manager` WHERE `booking_date` = CURDATE() -1");

return $results;
}



add_shortcode('name', 'reservations_yesterday');

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