In certain cases, you are not actually interested in the data themselves but
more in information about these data. For instance, you may want to know
the number of rows in a certain table. A number of functions are available in
SQL that help you to derive such information about the tables in your database.
The following aggregate functions can be used:

COUNT ( [column] )

counts the number of rows in a table

SUM ( [column] )

adds all the numerical values in the specified column

AVG ( [column] )

calculates the average of all the values in the specified
column

MIN ( [column] )

returns the minimum of all the numeric values in
the specified column

MAX ( [column] )

returns the maximum of all the numeric values in the specified
column

When you use one of these functions, the result of the query will contain one
row and one column only. This single cell contains the number that is the result
of the statistical function that has been applied. An example is given below.

SELECT COUNT(*)
FROM CITY;

This query will return one row and one cell which contains the number of cities
that are present in the table CITY.

COUNT() takes the name of a column as a paramter. This parameter should be
included within parentheses. The function will count all the records for which
the specified column is not empty. The best way to ensure that all the columns
are selected is by using the asterisk (*) as a parameter, as in the example above. Each record
in which any of the fields is not empty will be included in the count.

If you want to ensure that double values are not counted twice, you can of
course use DISTINCT in order to get rid of these double values before you start
counting. The following query counts the number of unique subject codes that
have been used in the table TREASURE.

SELECT COUNT(DISTINCT SUBJECT)
FROM TREASURE ;

The query below selects the oldest year of birth in the table CREATOR:

SELECT MIN(YEAR_OF_BIRTH)
FROM CREATOR ;

Exercises

Select the year of the oldest treasure in the database

Count the number of different countries in the table TREASURE

Find the most recent year of death in the table PERSON