So in the last quiz,
you saw one example of an aggregation,
a database operation that summarises
multiple rows into a single row.
Counting up the number of
gorillas in the animals table,
was a count aggregation.
It takes several rows,
each representing one gorilla, and
turns them into one gorilla
representing the count.
Any time we want to compute a single
result from a set of values,
that's an aggregation.
Here are some of the most common
aggregation functions in SQL.
Count is different from the other ones,
in that it takes values of any type, and
returns a number.
Most of the other aggregations
only work on numbers.
For instance, you can't the average
of a column of string values,
because only numbers have an average.
We'll see a lot more about
aggregations in the next lesson.