Question

I can’t figure out why the id returned with mysqli_insert_id() is different than my expected id. Here is my code:

$query = "INSERT INTO payments_table (payment_type, payment_number, payment_cost, insertion_date) VALUES ( '$paymentType', '$paymentNumber', '$totalCost', NOW())";
$data = mysqli_query($dbc, $query);
$currentPayerID = mysqli_insert_id($dbc);

//DEBUG
echo 'current payerID is: ' . $currentPayerID;

Since there are currently 2 rows, I expect it to echo “current payerID is 3” after the next execution. However, after executing this code, it echoes the following:
“current payerID is 21”

A couple notes:

1. This ‘payments_table’ table has a column named “payer_id” that is a
   primary key and has the AUTO_INCREMENT attribute.
2. I’ve “truncated” this table(I believe this is the same as emptying it out).

I searched throughout stackoverflow, but some of the answer don’t believe that what I’m finding is possible. I appreciate any generous help!

Answer 1

mysqli_insert_id returns the last AUTO_INCREMENT value – if you’re inserting to another table after payments_table but before calling mysqli_insert_id you may be getting an unexpected value.

Also an AUTO_INCREMENT column does not correspond with the number of rows in the database table, but the total number of rows over the lifetime of the table. More precisely, MySQL maintains a counter variable that is incremented each time a new row is created.

The following query will show the next expected AUTO_INCREMENT value for a table:

SELECT AUTO_INCREMENT FROM information_schema.TABLES WHERE TABLE_SCHEMA = 'my_database_name' AND TABLE_NAME = 'my_table_name';