The GROUPing pitfall
The GROUPing pitfall is a very common logical error in SQL queries, especially among people getting familiar with a DBMS.
This is what will happen - you'll be writing a query with a couple JOINs in it. You add a GROUP BY for the correct columns, and everything looks peachy. Then, at some point in the future (could be 5 minutes, could be a month), BOOM! Your query is returning random shit, and you have no clue why.
Considering that we here at ISoft pay the bills by writing queries that DON'T return random shit, this is something you should probably care about.
The short version
If your query joins on more than one table with a one-to-many relationship with the row you're grouping by, you CAN NOT use any aggregate functions in your query.
If you do, then any time that more than one of those JOINs matches more than one row, all of your aggregate functions will break.
Wait, why?
You know about JOINs, right? Let's say you have two tables:
| id (primary key) | value | weight |
|---|---|---|
| 1 | Bobo | 220 |
| 2 | Bimbo | 200 |
| 3 | The Hooker | 375 |
| id (primary key) | monkeyid | date | size |
|---|---|---|---|
| 1 | 2 | 2010-01-01 13:43:04 | 4.3 |
| 2 | 3 | 2010-01-02 06:12:44 | 3.8 |
| 3 | 1 | 2010-01-02 09:14:56 | 4.0 |
| 3 | 2 | 2010-01-02 15:05:33 | 2.6 |
As you can infer, there is a one-to-many relationship between "monkey" and "poop". That is to say, one monkey can have any number of poops.
I'm sure you understand what happens when they are joined together: Template:MySQL
You get this!
| monkey_name | poop_size |
|---|---|
| Bobo | 4.0 |
| Bimbo | 4.3 |
| Bimbo | 2.6 |
| The Hooker | 3.8 |
But you already knew that. I mean, that's kid stuff! I didn't even write it out, I actually got one of my younger siblings to type it out because I was so bored by it.
