691400. Query 3 returns a total of 397900, since many employees get paid the same
salary and the DISTINCT keyword only adds unique values in the column to the total.
Query 4 returns 7.8 after adding the non-null COMMISSION_PCT values.
The AVG Function
The average value of a column or expression is obtained by dividing the sum by the
number of non-null rows in the group. The AVG function has this syntax:
AVG([DISTINCT|ALL] expr) ;This syntax may be deconstructed into the following forms:
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