Querying from multiple tables

When data spans multiple tables, you need to be able to combine (or join) tables. The logic for this comes from set theory.

| A $\cap$ B | A $\cup$ B

Exercise: Joining data to make a query

Consider an example where two tables exist in a database. A superhero table collects information about the superhero names and date of birth. A powers table collects information about each individual's superpower information.

Let's say we want to query for all humans who are at least of drinking age (born before 12/31/1990). We would have to join both tables into one temporary big table before applying filter conditions.