LAST_INSERT_ID() is unequal to $db->insert_id?

Question

I have the following query:

$year         = 2019;
$month        = 6;
$stmt         = $db->prepare(\'INSERT INTO officeRechNr (jahr,monat,zahl) VALUES (?,?,1)          


        

Answer 1

The problem is that LAST_INSERT_ID(...); with an argument doesn't return the generated ID but instead set the given value in the "memory" of LAST_INSERT_ID() and returns it. So, in your first execution no auto incremented ID was generated (you provided the value by yourself) and LAST_INSERT_ID() return 0. In your following executions you save the value next+1 in the internal storage of LAST_INSERT_ID(), which returns the value. This behavior is described in the MySQL in 12.14 Information Functions:

If expr is given as an argument to LAST_INSERT_ID(), the value of the argument is returned by the function and is remembered as the next value to be returned by LAST_INSERT_ID().

In fact, you can skip the LAST_INSERT_ID() call and work without it.

INSERT INTO
    officeRechNr (jahr,monat,zahl)
VALUES
    (?,?,1)
ON DUPLICATE KEY UPDATE zahl = zahl+1

This will insert the row (with the given value) or increase the counter.

If you want the current counter for a given year and month you run a simple SELECT statement. Keep in mind that you might need transactions or locks because a different client could increase the counter before you fetched it with the SELECT statement.