Blog, page 22

Field notes — one a day since May 2023. Things we shipped, things we broke, and numbers we went and checked.

2026 2025 2024 2023

1,200 posts. , or browse by year and tag below.

Mar 22, 2025

Sessions out of raw events in saas-subscriptions

sessionisation against a real schema, and the clause in it that is doing the work.

sql Analytics engineering

Mar 21, 2025

Counting rows is not testing grain

3,780 distinct combinations against 4,000 rows, and what that answers about the table's key.

sql quality warehousing Analytics engineering

Mar 20, 2025

Five minutes with country in world-indicators

720 rows, 30 distinct values, and the one fact about country that changes how you query it.

quality practice Data quality

Mar 19, 2025

Before you clip the outliers

A max 1.1× the 99th percentile, and the three different things that could be.

statistics quality Statistics

Mar 18, 2025

seats_now: mean 59, median 8

A 635.5% gap between mean and median on one column, and which of the two answers the question you were asked.

statistics practice Statistics

Mar 17, 2025

What rated_at actually contains in movie-ratings

9,000 rows, 8,999 distinct values, and the one fact about rated_at that changes how you query it.

quality practice Data quality

Mar 16, 2025

The overall average hides 14 different numbers

Real group means for one column across 14 segments, and what the pooled average conceals.

statistics practice Analytics practice

Mar 15, 2025

r = -0.37 between wind_mw and price_eur_mwh

A measured correlation of -0.37 between two columns, what its square says, and the diagnostic that costs one line.

statistics Statistics

Mar 14, 2025

Charting seats by plan

Two facts about the columns decide the chart. Cardinality and skew, in that order.

visualization Visualization

Mar 13, 2025

Reading release_year before trusting it in movie-ratings

9,000 rows, 53 distinct values, and the one fact about release_year that changes how you query it.

quality practice Data quality

Mar 12, 2025

Retail orders, in one paragraph

Seven thousand e-commerce order lines across four channels and six categories, with a real Q4 seasonality bump, a returns flag, and discounting that varies by channel..

quality practice Data quality

Mar 11, 2025

Reader question — Writing SQL people can read, and reading the plan

The query you write twice a year is written for the machine. The one in a production model is written for the person debugging it at 3am, who might be you.

practice Analytics engineering

Mar 10, 2025

What dep_delay_min actually contains in flight-delays

8,000 rows, 251 distinct values, and the one fact about dep_delay_min that changes how you query it.

quality practice Data quality

Mar 9, 2025

Anti-join, in short

Find rows with no match on the other side — and avoid the NOT IN trap that silently returns nothing.

sql Analytics engineering

Mar 8, 2025

The category that eats the chart

4 values with 29.6% concentrated in one of them, and what that does to every chart downstream.

visualization practice Visualization

Mar 7, 2025

Keeping one row per key in support-tickets

deduplicate by rank against a real schema, and the clause in it that is doing the work.

sql Analytics engineering

Mar 6, 2025

Counting rows is not testing grain

7,690 distinct combinations against 8,000 rows, and what that answers about the table's key.

sql quality warehousing Analytics engineering

Mar 5, 2025

Five minutes with wind_mw in grid-energy-load

8,760 rows, 6,314 distinct values, and the one fact about wind_mw that changes how you query it.

quality practice Data quality

Mar 4, 2025

A strong correlation, and what it is not

A measured correlation of 0.80 between two columns, what its square says, and the diagnostic that costs one line.

statistics Statistics

Mar 3, 2025

Before you clip the outliers

A max 1.1× the 99th percentile, and the three different things that could be.

statistics quality Statistics

Mar 2, 2025

Five minutes with first_response_min in support-tickets

4,000 rows, 622 distinct values, and the one fact about first_response_min that changes how you query it.

quality practice Data quality

Mar 1, 2025

The overall average hides 8 different numbers

Real group means for one column across 8 segments, and what the pooled average conceals.

statistics practice Analytics practice

Feb 28, 2025

Charting fare_usd by borough

Two facts about the columns decide the chart. Cardinality and skew, in that order.

visualization Visualization

Feb 27, 2025

Reading sessions before trusting it in ab-test-checkout

6,000 rows, 20 distinct values, and the one fact about sessions that changes how you query it.

quality practice Data quality