PostgreSQL 9.2.0 Documentation | ||||
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Like most other relational database products, PostgreSQL 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)

If we wanted to know what city (or cities) that reading occurred in, we might try:

SELECT city FROM weather WHERE temp_lo = max(temp_lo);WRONG

but this will not work since the aggregate `max`

cannot be used in the `WHERE` clause. (This restriction exists because the
`WHERE` clause determines which rows will be
included in the aggregate calculation; so obviously it has to be
evaluated before aggregate functions are computed.) However, as is
often the case the query can be restated to accomplish the desired
result, here by using a *subquery*:

SELECT city FROM weather WHERE temp_lo = (SELECT max(temp_lo) FROM weather);

city --------------- San Francisco (1 row)

This is OK because the subquery is an independent computation that computes its own aggregate separately from what is happening in the outer query.

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. We can filter
these grouped rows using `HAVING`:

SELECT city, max(temp_lo) FROM weather GROUP BY city HAVING max(temp_lo) < 40;

city | max ---------+----- Hayward | 37 (1 row)

which gives us the same results for only the cities that have
all `temp_lo` values below 40. Finally,
if we only care about cities whose names begin with "`S`", we might do:

SELECT city, max(temp_lo) FROM weather WHERE city LIKE 'S%'(1) GROUP BY city HAVING max(temp_lo) < 40;

- (1)
- The
`LIKE`operator does pattern matching and is explained in Section 9.7.

It is important to understand the interaction between aggregates
and SQL's `WHERE` and `HAVING` clauses.
The fundamental difference between `WHERE`
and `HAVING` is this: `WHERE` selects input rows before groups and
aggregates are computed (thus, it controls which rows go into the
aggregate computation), whereas `HAVING`
selects group rows after groups and aggregates are computed. Thus,
the `WHERE` clause must not contain
aggregate functions; it makes no sense to try to use an aggregate
to determine which rows will be inputs to the aggregates. On the
other hand, the `HAVING` clause always
contains aggregate functions. (Strictly speaking, you are allowed
to write a `HAVING` clause that doesn't use
aggregates, but it's seldom useful. The same condition could be
used more efficiently at the `WHERE`
stage.)

In the previous example, we can apply the city name restriction
in `WHERE`, since it needs no aggregate.
This is more efficient than adding the restriction to `HAVING`, because we avoid doing the grouping and
aggregate calculations for all rows that fail the `WHERE` check.