MySQL Cookbook, 2nd Edition

Using Sequence Generators as Counters

Problem

You’re interested only in counting events, so there’s no point in creating a table row for each sequence value.

Solution

Use a sequence-generation mechanism that uses just one row per counter.

Discussion

AUTO_INCREMENTcolumns are useful for generating sequences across a set of individual rows. But for some applications, you’re interested only in a count of the number of times an event occurs, and there’s no value in creating a separate row for each event. Instances include web page or banner ad hit counters, a count of items sold, or the number of votes in a poll. For such applications, you need only a single row to hold the count as it changes over time. MySQL provides a mechanism for this that enables counts to be treated like AUTO_INCREMENT values so that you can not only increment the count, but retrieve the updated value easily.

To count a single type of event, you can use a trivial table with a single row and column. For example, if you’re selling copies of a book, you can create a table to record sales for it like this:

CREATE TABLE booksales (copies INT UNSIGNED);

However, if you’re counting sales for multiple book titles, that method won’t work so well. You certainly don’t want to create a separate single-row counting table per book. Instead, you can count them all within a single table if you include a column that provides a unique identifier for each book. The following table, booksales, does this using a title column for the book ...