There’s a MySQL pattern that I use fairly frequently that I want to share, both to help anyone else who might find it useful and also to find out if there’s a beter way.

Here’s an example: you have one table for users, another for posts, another for pages:

And we want to know how many posts and pages each user has:

Attempt 1: COUNT with JOIN

For the moment, lets focus on just getting the post count by user.

We might be tempted to try JOINing the two tables and using COUNT:

The problem is that when we JOIN the two tables, the results will only include users who have posts. In this case, there’s no result for Jen (user id 3) because she doesn’t have any records in the posts table.

Attempt 2: COUNT with LEFT JOIN

Ok, so if JOIN doesn’t work, lets try LEFT JOIN:

Now Jen has a count, but it’s reflecting 1 post, not 0 like we expect.

We can see why it’s broken by looking at the LEFT JOIN results without aggregating them:

Jen doesn’t have any posts, but because we’re LEFT JOINing, her users record is still included. When we then aggregate the results with GROUP BY and COUNT, MySQL sees that the results have one record so returns a count of 1.

A better way to do this (as suggested by Tom Davies) is instead of counting all records, only count post ids:

Attempt 3: SUM/IF, and LEFT JOIN

Another way we can achieve what we want (for a single table join) is to use SUM/IF:

Here we’re saying “When the post id is null, give it a 0, otherwise a 1, then sum the results” which gives us the correct count. This is actually I trick I learned from the real Jen while at Automattic :).

The problem arises when we need to join multiple tables:

Now instead of Simon (user id 2) having 2 posts, he has 4 – what happened? Again, we can look at the un-aggregated results:

The problem is that each post gets joined with each page result. Simon has 2 posts, each of which gets joined with the 2 pages, so when we use COUNT it sees 4 results and returns that amount.

The solution: Subqueries and COALESCE

Here’s how I would solve it:

To understand how it works, lets focus on counting posts. The first subquery counts how many posts each user has if they have any posts:

We can then LEFT JOIN users on this derived table:

For the users with posts, the result has the post count. For the users without posts, the result is NULL. This is where COALESCE comes into play. COALESCE takes any number of arguments and returns the first non-NULL result:

So we’re saying “If the user has a posts count, use that, otherwise use 0”.

We can then LEFT JOIN again on the pages table and do the same thing with the pages count to get the posts and pages count per user. Because each subquery only returns a max of one result per user, we don’t run into the issue we did earlier where posts get joined with pages to return the incorrect count.

Somewhat complicated, but the only way I know how to do it. If you know of a better way, please drop a comment below. Thanks!

Edit:

This simpler method also works:

By counting distinct post ids and page ids, we avoid counting NULLs and also avoid counting duplicates due to joining on posts and pages.

In my last post, I wrote about how to set up a Bash alias to pull your Heroku data into a local Postgres database. This post takes it a step further by showing how to automate it so that the database is automatically updated on a regular basis.

Background

We left off the last post with this Bash alias:

alias prdb="dropdb preceden_production_copy; PGUSER=postgres PGPASSWORD=password heroku pg:pullHEROKU_POSTGRESQL_MAGENTA preceden_production_copy --app sushi"

This lets me run prdb which first deletes my local database copy, then pulls the production data into a new one.

When attempting to schedule this as a cronjob, I ran into several issues.

First, dropdb won’t work if the database is being used by any applications. You’ll get an error message like:

dropdb: database removal failed: ERROR:  database "preceden_production_copy" is being accessed by other users

This is a problem because I usually leave Metabase and SQLPro for Postgres open which causes dropdb to throw and error which causes pg:pull not to work because the database already exists. I then have to shut down the apps and try again, and even then there’s often some hidden connection that requires a complete reboot.

Second, it usually takes about half an hour for pg:pull to load all of the data into a database. If I’m eager to dive into the latest data, it’s frustrating to have to wait that long to do it.

Lastly, you can’t use aliases in a cronjob, meaning you’d have to duplicate the content of the alias in the cronjob.

As we’ll see, we can work through each of these issues.

Solution

There’s a way with Postgres to drop existing connections. By excuting this query through terminal before we attempt to drop the database, we ensure the command will work. Here’s what it looks like:

echo "SELECT pg_terminate_backend(pid) FROM pg_stat_activity WHERE datname='preceden_production_copy';" | psql -U postgres

If all goes well, you’ll see something like:

pg_terminate_backend 
----------------------
 t
 t
(2 rows)

Next, in order to avoid duplicating the alias command and the cronjob command, we’ll combine all of the commands into a Bash script which we’ll execute from both places:

#!/bin/bash

current_date_time="`date +%Y\-%m\-%d\ %H\:%M\:%S`";
echo "Running at:" $current_date_time;

echo "SELECT pg_terminate_backend(pid) FROM pg_stat_activity WHERE datname='preceden_production_copy';" | psql -U postgres
dropdb preceden_production_copy
PGUSER=postgres PGPASSWORD=password heroku pg:pullHEROKU_POSTGRESQL_MAGENTA preceden_production_copy --app sushi

This will output the date and time when the script is being run, then drop any open database connections, remove the database, then pull the production Postgres data into a new database.

Next, we can update the alias to point to this script:

alias prdb="/Users/matt/Projects/Preceden/prdb.sh"

And we can point to it from a cronjob:

0 12 * * * /Users/matt/Projects/Preceden/prdb.sh >> /Users/matt/Projects/Preceden/prdb.log 2>&1

This says “At noon every day, run prdb.sh and append the output to prdb.log”.

And voila! The data should get updated automatically once a day thanks to the cronjob and you still have the ability to run it manually using the alias.