2.7. Aggregate Functions

Like most other relational database products, Postgres-XC supports aggregate functions. An aggregate function computes a single result from multiple input rows. For example, there are aggregates to compute the count, sum, avg (average), max (maximum) and min (minimum) over a set of rows.

As an example, we can find the highest low-temperature reading anywhere with:

SELECT max(temp_lo) FROM weather;

 max
-----
  46
(1 row)

Aggregates are also very useful in combination with GROUP BY clauses. For example, we can get the maximum low temperature observed in each city with:

SELECT city, max(temp_lo)
    FROM weather
    GROUP BY city;

     city      | max
---------------+-----
 Hayward       |  37
 San Francisco |  46
(2 rows)

which gives us one output row per city. Each aggregate result is computed over the table rows matching that city.