Master one of the great open source databases.

Learning SQL was one of the most valuable skills I learned while building Match.  We had one of the largest instances of Microsoft SQL Server in the 90s with millions of records to keep the site running.  The better we got at SQL, the better we could make the site work for our users and answers sophisticated questions about our users. 

I want to teach you how to use PostgreSQL.  We will walk through

Basic selection statements
Joining multiple tables together
Grouping records to get aggregate data
Inserting, updating and deleting records
Creating tables and indexes
Constraining data input using field and table constraints