Fix table increment counter in MariaDB or MySQL after manual row deletion

Written by Claudio Kuenzler - 0 comments

Published on July 24th 2018 - Listed in PHP MySQL MariaDB Database

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I recently upgraded this tech blog from PHP 5.6 to 7.0 and stumbled (again) across some old mysql* functions. These were removed in PHP 7.0 and needed to be replaced by either PDO or MySQLi (see Changing from PHP's mysql to myqli - what to look at).

While I fixed most of the code, I forgot the admin part of my blog. Before a new article is inserted into the database, the content/text runs through a function to escape special characters: mysqli_real_escape_string(). From the documentation:

"Escapes special characters in a string for use in an SQL statement, taking into account the current charset of the connection"

Old mysql_real_escape_string allowed to simply use a single variable:

# OLD PHP < 7
$iContent = mysql_real_escape_string($iContent);

But (almost all) mysqli functions require the mysqli connection variable (here $connect), too:

# NEW PHP >= 7
$iContent = mysqli_real_escape_string($connect, $iContent);

Long story short: The content was not inserted into the database and once I fixed the code, I had to delete my prior attempts in the table and manually update the article ID to not leave a gap in between articles. This had a negative impact on the table's auto increment counter.

To better show that, I retrieve the latest article ID:

MariaDB [claudiokuenzler]> select newsid from news order by newsid desc limit 0,1;
+--------+
| newsid |
+--------+
|    790 |
+--------+
1 row in set (0.00 sec)

Yet the auto increment counter was already at 793 for the next insert (I manually deleted 2 entries):

MariaDB [(claudiokuenzler)]> SELECT `AUTO_INCREMENT` FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA = 'claudiokuenzler' AND TABLE_NAME = 'news';
+----------------+
| AUTO_INCREMENT |
+----------------+
|            793 |
+----------------+
1 row in set (0.00 sec)

Of course I wanted to fix this immediately and luckily I came across this stackoverflow question where user Anshul gave a very good and quick explanation:

Further, in order to reset the AUTO_INCREMENT count, you can immediately issue the following statement.
ALTER TABLE `users` AUTO_INCREMENT = 1;
For MySQLs it will reset the value to MAX(id) + 1.

So I did that:

MariaDB [claudiokuenzler]> ALTER TABLE news AUTO_INCREMENT = 1;
Query OK, 788 rows affected (0.01 sec)            
Records: 788  Duplicates: 0  Warnings: 0

And how did this affect the increment counter?

MariaDB [claudiokuenzler]> SELECT `AUTO_INCREMENT` FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA = 'claudiokuenzler' AND TABLE_NAME = 'news';
+----------------+
| AUTO_INCREMENT |
+----------------+
|            791 |
+----------------+
1 row in set (0.00 sec)

Yes! The next insert will have the next ID of 791. Hurray.

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