Contents /
Previous /
Next
Aggregate Functions
An aggregate function computes a single result from multiple input rows.
Five popular aggregate functions are:
| Aggregate |
Function |
| COUNT(*) |
count of rows |
| SUM(colname) |
total |
| MAX(colname) |
maximum |
| MIN(colname) |
minimum |
| AVG(colname) |
average |
For example, count the students and calculate the average semester:
<![CDATA[
> SELECT count(semester) FROM stud ;
count
-------
4
(1 row)
> SELECT avg(semester) FROM stud;
avg
--------------
2.0000000000
(1 row)
]]>
NULL values are not processed by most aggregates, such as MAX(),
SUM(), and AVG(); they are simply ignored.
COUNT(*) is different in this respect. It does count NULL values because it looks
at entire rows using the asterisk(*).