Rather than using the raw primary key values of our instruments table each time, we can use a subquery as shown in the following SQL query:
UPDATE musicians SET main_instrument=(SELECT id FROM instruments WHERE name='guitar') WHERE name IN ('Robert Fripp', 'David Gilmour');
A subquery is a SQL query within a SQL query. If your subquery can be guaranteed to return a single value, it can be used anywhere you would use a literal value. In this case, we're letting our database do the work of figuring out what the primary key of guitar is, and inserting that for our main_instrument value.
In the WHERE clause, we've also used the IN operator to match against a list of values. This allows us to match against a list of values.
IN can be ...
