Jon Shaulis shows why you want to reference tables when including column names in queries:
If you don’t read the rest of this setup, I want you to take away one thing.
Always reference your tables with your columns when more than one table is involved in the query!
This post is made primarily with SQL Server in mind, but this behavior is actually ANSI SQL and can be replicated in PostgreSQL, MySQL, and Oracle.
Jon’s example is a case where perfectly valid ANSI SQL logic (which is why you can replicate this across platforms and why it’s not a bug) leads to an unexpected result.