Had a great opportunity this weekend to teach an introductory SQL class to about 50 MBA students. We focused a lot on queries needed for business analysis, including aggregation queries.
One of the key topics that probably isn’t the most intuitive for a newer database person is the difference between filtering results with WHERE and filtering results with HAVING.
Do you have the correct (data) filter?
In a nutshell, a WHERE clause filters data before aggregation takes place, while HAVING filters the results based on the aggregation.
Assume you have a database table with 200,000 records. A WHERE clause will filter those down, let’s say, to 125,000 records, then your GROUP BY will aggregate those 125,000 records.
Without a WHERE clause, though, all 200,000 records will be aggregated, then the HAVING condition at the end of the GROUP BY is applied to the aggregated values.
Want a more thorough walk-through with sample data? Just click here.
And as long as you’re here, how about a little shameless advertising? This week I completed a totally different project. My first card game, “Steal This Election!” became available via The Game Crafter. If pandemics and politics have left you in need of a laugh, check it out.