The AVG function

The following query computes the average unit_price of all rows in the stock table.
Figure 1: Query
SELECT AVG (unit_price) FROM stock;
Figure 2: Query result
  (avg)

$197.14
The following query computes the average unit_price of just those rows in the stock table that have a manu_code of SHM.
Figure 3: Query
SELECT AVG (unit_price) FROM stock WHERE manu_code = 'SHM';
Figure 4: Query result
  (avg)

$204.93