Simple aggregation

This is our first foray into SQL's aggregate functions. They're used to extract information about whole groups of rows, and allow us to easily ask questions like:

• What's the most expensive facility to maintain on a monthly basis?
• Who has recommended the most new members?
• How much time has each member spent at our facilities?

The MAX aggregate function here is very simple: it receives all the possible values for joindate, and outputs the one that's biggest. There's a lot more power to aggregate functions, which you will come across in future exercises.