MySQL Cookbook

11.1 Introduction

A sequence is a set of integers 1, 2, 3, ... that are generated in order on demand. Sequences are frequently used in databases because many applications require each row in a table to contain a unique value, and sequences provide an easy way to generate them. This chapter describes how to use sequences in MySQL. It covers the following topics:

• Using AUTO_INCREMENT columns to create sequences.

  The AUTO_INCREMENT column is MySQL's mechanism for generating a sequence over a set of rows. Each time you create a row in a table that contains an AUTO_INCREMENT column, MySQL automatically generates the next value in the sequence as the column's value. This value serves as a unique identifier, making sequences an easy way to create items such as customer ID numbers, shipping package waybill numbers, invoice or purchase order numbers, bug report IDs, ticket numbers, or product serial numbers.

• Retrieving sequence values.

  For many applications, it's not enough just to create sequence values. It's also necessary to determine the sequence value for a just-inserted record. A web application may need to redisplay to a user the contents of a record created from the contents of a form just submitted by the user. Or the value may need to be retrieved so it can be stored as part of other records in a related table.

• Resequencing techniques.

  This section describes how to renumber a sequence that has holes in it due to record deletions—and also discusses reasons to avoid resequencing. Other topics include starting sequences at values other than 1 and adding a sequence column to a table that doesn't have one.

• Using an AUTO_INCREMENT column to create multiple sequences.

  In many cases, the AUTO_INCREMENT column in a table is independent of other columns and its values increment throughout the table in a single monotonic sequence. However, if you create a multiple-column index that contains an AUTO_INCREMENT column, you can use it to generate multiple sequences. For example, if you run a bulletin board that categorizes messages into topics, you can number messages sequentially within each topic by tying an AUTO_INCREMENT column to a topic indicator column.

• Managing multiple simultaneous AUTO_INCREMENT values.

  Special care is necessary when you need to keep track of multiple sequence values. This can occur when you issue a set of statements that affect a single table, or when creating records in multiple tables that each have an AUTO_INCREMENT column. This section describes what to do in these cases.

• Using single-row sequence generators.

  Sequences also can be used as counters. For example, if you serve banner ads on your web site, you might increment a counter for each impression (that is, for each time you serve an ad). The counts for a given ad form a sequence, but because the count itself is the only value of interest, there is no need to generate a new row to record each impression. MySQL provides a solution for this problem, too, using a mechanism that allows a sequence to be easily generated within a single table row over time. To store multiple counters in the table, add a column that identifies the counter uniquely. For example, you can have an arbitrary number of ad impression counters in a table. Each row in the table identifies a specific banner ad, and the counter in each row increments independently of the others. The same mechanism also allows creation of sequences that increase by values other than one, by non-uniform values, or even by negative increments.

• Numbering query output rows sequentially.

  This section suggests ways to generate display-only sequences for the purpose of numbering the rows of output from a query.

Sequence Generators and Portability The engines for most database systems provide sequence generation capabilities, though the implementations tend to be engine-dependent. That's true for MySQL as well, so the material in this section is almost completely MySQL-specific, even at the SQL level. In other words, the SQL for generating sequences is itself non-portable, even if you use an API like DBI or JDBC that provides an abstraction layer. Abstract interfaces may help you process SQL statements portably, but they don't make nonportable SQL portable.