

PostgreSQL 8.1.4 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 lowtemperature 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 the rows that will
go into the aggregation stage; so it has to be evaluated before
aggregate functions are computed.)
However, as is often the case
the query can be restated to accomplish the intended 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;
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.