How can I make Excel fetch data from a database automatically when I open the spreadsheet?

Of course. Here are the basics.

1) Create a connection (once)

• Excel-Data-From Other Sources-SQL Server (this is for Microsoft SQL, but other data sources are available)
• Enter Server credentials, uncheck "Connect to a specific table" if you wish to use various tables or SQL queries later on.
• Finish. It brings you automatically to the step below, but you don't have to repeat that part.

2) Add connection to workbook

• Excel-Data-Existing Connections
• Pick connection you just saved
• Pick a table. If you wish to use a query, pick any table, then on the next screen (Import Data), click Properties, Definition tab, change Command type from Table to SQL and paste your query below.
• Import as Table

3) List and modify connections at anytime

• Excel-Data-Connections
• You can refresh, delete and also access Properties for example to modify the SQL queries without adding a new connection.
• Here in properties you can also set options to refresh automatically and at startup.