Question: I know you can ALTER the column order in MySQL with FIRST and AFTER, but why would you want to bother? Since good queries explicitly name columns when inserting data, is there really any reason to care what order your columns are in in the table?

Answer: 
**Update:**

In `MySQL`, there may be a reason to do this.

Since variable datatypes (like `VARCHAR`) are stored with variable lengths in `InnoDB`, the database engine should traverse all previous columns in each row to find out the offset of the given one.

The impact may be as big as **17%** for `20` columns.

See this entry in my blog for more detail:

* [**Choosing column order**](http://explainextended.com/2009/05/21/choosing-column-order/)

In `Oracle`, trailing `NULL` columns consume no space, that's why you should always put them to the end of the table.

Also in `Oracle` and in `SQL Server`, in case of a large row, a `ROW CHAINING` may occur.

`ROW CHANING` is splitting a row that doesn't fit into one block and spanning it over the multiple blocks, connected with a linked list.

Reading trailing columns that didn't fit into the first block will require traversing the linked list, which will result in an extra `I/O` operation.

See [**this page**](http://www.akadia.com/services/ora_chained_rows.html) for illustration of `ROW CHAINING` in `Oracle`:

That's why you should put columns you often use to the beginning of the table, and columns you don't use often, or columns that tend to be `NULL`, to the end of the table.

**Important note:**

If you like this answer and want to vote for it, please also vote for [`@Andomar`'s answer](https://stackoverflow.com/questions/894522/is-there-any-reason-to-worry-about-the-column-order-in-a-table/894545#894545).

He answered the same thing, but seems to be downvoted for no reason.
As is often the case, the biggest factor is the next guy who has to work on the system. I try to have the primary key columns first, the foreign key columns second, and then the rest of the columns in descending order of importance / significance to the system.