MySQL Cookbook

Avoiding Output Column Order Problems When Writing Programs

Problem

You’re issuing a SELECT * query from within a program, and the columns don’t come back in the order you expect.

Solution

When you use * to select columns, all bets are off; you can’t assume anything about the order in which they’ll be returned. Either name the columns explicitly in the order you want, or retrieve them into a data structure that makes their order irrelevant.

Discussion

The examples in the previous section illustrate the differences between using * versus a list of names to specify output columns when issuing SELECT statements from within the mysql program. The difference between approaches also may be significant when issuing queries through an API from within your own programs, depending on how you fetch result set rows. If you select output columns using *, the server returns them using the order in which they are listed in the table definition—an order that may change if the table structure is modified. If you fetch rows into an array, this non-determinacy of output column order makes it impossible to know which column each array element corresponds to. By naming output columns explicitly, you can fetch rows into an array with confidence that the columns will appear in the array in the same order that you named them in the query.

On the other hand, your API may allow you to fetch rows into a structure containing elements that are accessed by name. (For example, in Perl you can use a hash; in PHP ...