SQL Can't Delete from Table - php

Question

I have a PHP and I want to do 2 inserts and 1 delete, but I can only make an insert. If the array containt the last parameter == "historico" should delete from instant_table all register with same serial_num and inserte the array intro the instant_table and insert in historical_table("SensorData"). Ifnot (the array don't hace the parameter "historico"), should de delete from instant_table all register with same serial_num and only inserte the array intro the instant_table.
My code:
if ($_SERVER["REQUEST_METHOD"] == "POST") {
$serial_numb = test_input($_POST["serial_numb"]);
$DHTtempC = test_input($_POST["DHTtempC"]);
$DHThumid = test_input($_POST["DHThumid"]);
$CCS811_CO2 = test_input($_POST["CCS811_CO2"]);
$CCS811_tVOC = test_input($_POST["CCS811_tVOC"]);
$PM25 = test_input($_POST["PM25"]);
$PM10 = test_input($_POST["PM10"]);
$reading_date = date("Y-m-d");
$update_status = test_input($_POST["update_status"]);
$tipo_tabla = test_input($_POST["tipo_tabla"]);
// Create connection
// Check connection
if ($mysqli->connect_error) {
die("Connection failed: " . $mysqli->connect_error);
}
if ($tipo_tabla == "historico"){
$sql = "INSERT INTO SensorData (serial_numb, DHTtempC, DHThumid, CCS811_CO2, CCS811_tVOC, PM25, PM10, reading_date, update_status)
VALUES ('" . $serial_numb . "', '" . $DHTtempC . "', '" . $DHThumid . "', '" . $CCS811_CO2 . "', '" . $CCS811_tVOC . "', '" . $PM25 . "', '" . $PM10 . "', '" . $reading_date . "', '" . $update_status . "')";
}
$sql = "DELETE FROM instant_data WHERE (serial_numb = '" . $serial_numb . "')";
$sql = "INSERT INTO instant_data (serial_numb, DHTtempC, DHThumid, CCS811_CO2, CCS811_tVOC, PM25, PM10, reading_date, update_status)
VALUES ('" . $serial_numb . "', '" . $DHTtempC . "', '" . $DHThumid . "', '" . $CCS811_CO2 . "', '" . $CCS811_tVOC . "', '" . $PM25 . "', '" . $PM10 . "', '" . $reading_date . "', '" . $update_status . "')";
if ($mysqli->query($sql) === TRUE) {
echo "New record created successfully";
}
else {
echo "Error: " . $sql . "<br>" . $mysqli->error;
}
if ($mysqli->connect_error) {
die("Connection failed: " . $mysqli->connect_error);
}
$mysqli->close();
}
else {
echo "No data posted with HTTP POST.";
}
function test_input($data) {
$data = trim($data);
$data = stripslashes($data);
$data = htmlspecialchars($data);
return $data;
}
Tu sum up, If the array contains the parameter, INSERTE(TABLE1) + DELETE with same serial_num(TABLE2) + INSERTE(TABLE2). If not DELETE with same serial_num(TABLE2) + INSERTE(TABLE2).
EDIT: Now this code only make the second INSERT

Answer 1

It seems like you are overwriting the content of $sql without executing the queries in between. You have to either:
execute each query before redefining $sql
use $sql .= (instead of $sql =) to concatenate the next query. If you do this, you have to terminate your sql query with an ; before concatenating the next query.
Are you using this code just for an small personal project or are you going to publish this in any way? In case of the later one:
please read into PHP SQL best practices. With your current approach you are vulnerable to SQL injections and your code is kinda difficult to read.