Looping mulitple SQL queries in multiple arrays - php

Question

I'm a PHP beginner. I've dabbled before, but when I've begun to get the knack a new project takes me elsewhere. If anyone can assist; I'd appreciate it.
I am using multiple queries and arrays to retrieve data between two mySQL tables starting from 1 initial known variant.
I'd like each query to process all results from the previous query. This is not occurring.
Current results: The first query echos all results. The second query echos one result. The third query echos 1 result. The final echo displays all the final results (desired, but missing the first 148 rows).
Desired results: Echo all 149 results from all three queries then echo a table/array of all 3 queries (to confirm correlation).
<?php
// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($conn->connect_error) {
die("Connection failed: " . $conn->connect_error);
}
//Select all POST_IDs for variation 2.1M
$sql = "SELECT post_id FROM wp_postmeta WHERE meta_value = '2-1m'";
$result = $conn->query($sql);
//Array and display POST_IDs
if ($result->num_rows > 0) {
// output data of each row
while($row = $result->fetch_assoc()) {
echo "id: " . $row["post_id"]. "<br>";
}
} else {
echo "0 results";
}
//Prepare POST_IDs for next query
foreach ($result as $row){
$postids = $row["post_id"];
}
//Use POST_IDs to select all PARENT_IDs
$sql2 = "SELECT post_parent FROM wp_posts WHERE ID = ($postids)";
$result2 = $conn->query($sql2);
//Array and display PARENT_IDs
if ($result2->num_rows > 0) {
// output data of each row
while($row2 = $result2->fetch_assoc()) {
echo "parentid: " . $row2["post_parent"]. "<br>";
}
} else {
echo "0 results";
}
//Prepare PARENT_IDs for next query
foreach ($result2 as $row2){
$parentids = $row2["post_parent"];
}
//Select PRICES using PARENT_IDs and META_KEY for Price
$sql3 = "SELECT meta_value FROM wp_postmeta WHERE meta_key = '_price' AND post_id = ($parentids)";
$result3 = $conn->query($sql3);
if ($result3->num_rows > 0) {
// output data of each row
while($row3 = $result3->fetch_assoc()) {
echo "price: " . $row3["meta_value"]. "<br>";
}
} else {
echo "0 results";
}
//Array and display PRICES
foreach ($result3 as $row3){
$prices = $row3["meta_value"];
}
//Display all retrieved data
echo "<div><p>" . $postids . " " . $parentids . " " . $prices . "</p></div>";
$conn->close();
?>

Answer 1

You're overriding your variables instead of cumulate them into an array:
foreach ($result as $row){
$postids = $row["post_id"];
}
Should be :
$postids = [];
foreach ($result as $row){
$postids[] = $row["post_id"];
}
Then :
"WHERE ID = ($postids)"
Should be :
if (!empty($postids)) {
... "...WHERE ID IN (".implode(',', $postids).")..." ...
}
NB: you should have a look to parameterized queries : Parameterized Queries PHP/MySQL
The same thing happend for $parentids.