Entries tagged with sql

A Tour of Tagging Schemas: Many-to-many, Bitmaps and More


In this post I'll describe how to implement tagging with a relational database. What I mean by tagging are those little labels you see at the top of this blog post, which indicate how I've chosen to categorize the content. There are many ways to solve this problem, and I'll try to describe some of the more popular methods, as well as one unconventional approach using bitmaps. In each section I'll describe the database schema, try to list the benefits and drawbacks, and present example queries. I will use Peewee ORM for the example code, but hopefully these examples will easily translate to your tool-of-choice.


Querying the top N objects per group with Peewee ORM


This post is a follow-up to my post about querying the top related item by group. In this post we'll go over ways to retrieve the top N related objects by group using the Peewee ORM. I've also presented the SQL and the underlying ideas behind the queries, so you can translate them to whatever ORM / query layer you are using.

Retrieving the top N per group is a pretty common task, for example:

  • Display my followers and their 10 most recent tweets.
  • In each of my inboxes, list the 5 most recent unread messages.
  • List the sections of the news site and the three latest stories in each.
  • List the five best sales in each department.

In this post we'll discuss the following types of solutions:

  • Solutions involving COUNT()
  • Solutions involving LIMIT
  • Window functions
  • Postgresql lateral joins


Querying the top item by group with peewee ORM


In this post I'd like to share some techniques for querying the top item by group using the Peewee ORM. For example,

  • List the most recent tweet by each of my followers.
  • List the highest severity open bug for each of my open source projects.
  • List the latest story in each section of a news site.

This is a common task, but one that can be a little tricky to implement in a single SQL query. To add a twist, we won't use window functions or other special SQL constructs, since they aren't supported by SQLite. If you're interested in finding the top N items per group, check out this follow-up post.


The search for the missing link: what lies between SQL and Django's ORM?

I had the opportunity this week to write some fairly interesting SQL queries. I don't write "raw" SQL too often, so it was fun to use that part of my brain (by the way, does it bother anyone else when people call SQL "raw"?). At Counsyl we use Django for pretty much everything so naturally we also use the ORM. Every place I've worked there's a strong bias against using SQL when you've got an ORM on board, which makes sense -- if you choose a tool you should standardize on it if for no other reason than it makes maintenance easier.

So as I was saying, I had some pretty interesting queries to write and I struggled to think how to shoehorn them into Django's ORM. I've already written about some of the shortcomings of Django's ORM so I won't rehash those points. I'll just say that Django fell short and I found myself writing SQL. The queries I was working on joined models from very disparate parts of our codebase. The joins were on values that weren't necessarily foreign keys (think UUIDs) and this is something that Django just doesn't cope with. Additionally I was interested in aggregates on calculated values, and it seems like Django can only do aggregates on a single column.

As I was prototyping, I found several mistakes in my queries and decided to run them in the postgres shell before translating them into my code. I started to think that some of these errors could have been avoided if I could find an abstraction that sat between the ORM and a string of SQL. By leveraging the python interpreter, the obvious syntax errors could have been caught at module import time. By using composable data structures, methods I wrote that used similar table structures could have been more DRY. When I write less code, I think I generally write less bugs as well.

That got me started on my search for the "missing link" between SQL (represented as a string) and Django's ORM.