MySQL SELECT sometimes found nothing even it exists - php

Question

I have PHP script that runs every 3 hours that gets the amazon mws orders and save it into my database.
But before I insert an order into my table, I check first if the order exist
SELECT COUNT(*) total FROM orders WHERE transaction_id = "XXXXXXXXX"
But sometimes even if an order exist, the query will return 0. If return 0, I will insert the order into my table and it will end up duplicating the data.
This instance happens to me in my other PHP scripts and I don't understand the issue.

Answer 1

You should use something called as EXISTS for checking whether a row exists or not in your case order.
SELECT EXISTS(SELECT * FROM table1 WHERE ...);
SELECT EXISTS(SELECT * FROM tableName WHERE condition);

Answer 2

If your database uses master-slave mode，you can try to force the master to query.
You can also use the “INSERT IGNORE INTO orders ....” to ignore duplicate.

Answer 3

You may have an error in your MySQL query. I created a test table and write some code to print the result of the query which is working correctly.
You may not be using the quotations properly as this is the error most
of the time when you are working with queries in PHP
Here is the code
<?php
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "practise";
// Create connection
$conn = mysqli_connect($servername, $username, $password, $dbname);
// Check connection
if (!$conn) {
die("Connection failed: " . mysqli_connect_errno());
}
$sql = "SELECT count(*) from test where title='Hello'";
$result = mysqli_query($conn,$sql);
$new =mysqli_fetch_array($result);
echo $new[0];
?>
</body>
</html>
And:
Image of the table where I am executing the query
The output against the MySQL query is:
2
The code works perfectly. If you have any other question, please share your code and I'll be more than happy to help you.