#
Funny SQL bug because I thought `row_number()`

started at 0

I was working on a toy project of mine and ran into a funny SQL bug because of an off-by-one error.

Can you spot the error in this SQL docstring?

```
/* Aggregate features to arrays of size sequence_length (default 100) per user.
*
* userid, floor(row_number/100), feat1, feat2, feat3
* user1, 0 , [...], [...], [...]
* user1, 1 , [...], [...], [...]
* user2, 0 , [...], [...], [...]
*
* Define bucket_num = floor(row_number/100). This maps row 1-100 to 0, row
* 101-200 to 1, etc. Finally, ARRAY_AGG and GROUP BY the userid *and* the
* bucket_num.
* /
```

This docstring is part of my data prep routine that aggregates a row-based SQL dataset into feature arrays to use as input for my machine learning model.

First, I assign a number to each observation per user `[0, 1, 2, ...]`

```
select row_number() over (partition by userid) as row_number
```

Then, I create a variable that maps these numbers into buckets of 100 observations each

```
select floor(row_number/100) as bucket_num
```

Finally, I group the observations with the same userid and bucket number together and aggregate them into arrays

```
select array_agg(feature1) as feat1
...
group by userid, bucket_num
having count(*)=100
```

This is how I thought it was going to work:

userid | row | row_number | floor(row_number/100) |
---|---|---|---|

1 | 1 | 0 | 0 |

1 | 100 | 99 | 0 |

1 | 101 | 100 | 1 |

But when I started validating both datasets the aggregated dataset with feature arrays was missing 15% of users! What!? Weird.

So, what do you do here? I started out by trying some low-hanging fruit, mostly
stray `where`

clauses. But none of them really seemed to hit the mark. However, once I started looking at the behavior of individual users everything started to make sense.

This is some data for a random user in the dataset that Iโm using.

The first column is the userid, the second column is the `floor(row_number/100)`

and the third column is a count of the rows in each bucket.

Surprisingly, we see that **the first bucket only has 99 entries instead of the expected 100**.

This anomaly showed immediately what was going wrong: **Because the row number doesnโt start at 0 but starts at 1, we filter out users that have at max 2 sequences**.

For example: Imagine a user with 150 rows in the dataset, because row number starts at 0 and not 1 we only map the first 99 rows to bucket 0 (1 to 99 is 99 numbers) and the remaining 51 rows to bucket 1. Both of these sequences are smaller than the required length of 100 and get filtered out by the `having count(*)=100`

clause.

We filter out both rows and *this drops the whole user!*

Thankfully, this is easy to fix. We simply subtract 1 from the row number to make the first bucket contain 100 elements:

userid | row | row_number-1 | floor((row_number-1)/100) |
---|---|---|---|

1 | 1 | 0 | 0 |

1 | .. | .. | .. |

1 | 100 | 99 | 0 |

1 | 101 | 100 | 1 |

Now we do not filter out users with only 2 sequences anymore because the first sequence, now with exactly 100 observatations, stays in the dataset. Problem solved!

Here is the corrected SQL snippet. The only difference is that we subtract 1 from the row number.

```
/* Aggregate features to arrays of size sequence_length (default 100) per user.
*
* userid, floor((row_number-1)/100), feat1, feat2, feat3
* user1, 0 , [...], [...], [...]
* user1, 1 , [...], [...], [...]
* user2, 0 , [...], [...], [...]
*
* Define bucket_num = floor((row_number-1)/100). This maps row 1-100 to 0, row
* 101-200 to 1, etc. Finally, ARRAY_AGG and GROUP BY the userid *and* the
* bucket_num.
* /
```

If you fiddle with row numbers, make sure to check whether they start at zero or one!

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