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Hello and welcome.
In this video, we'll briefly present a few useful expressions that
are used with select statements.
The first one is COUNT, COUNT is a built-in database function that
retrieves the number of rows that match the query criteria.
For example, get the total number of rows in a given table,
select COUNT(*) from tablename.
Let's say you create a table called MEDALS which has a column called COUNTRY, and
you want to retrieve the number of rows where the medal recipient is from Canada.
You can issue a query like this:
Select COUNT(COUNTRY) from MEDALS where COUNTRY='CANADA.'
The second expression is DISTINCT.
DISTINCT is used to remove duplicate values from a result set.
Example, to retrieve unique values in a column,
select DISTINCT columnname from tablename.
In the MEDALS table mentioned earlier,
a country may have received a gold medal multiple times.
Example, retrieve the list of unique countries that received gold medals.
That is, removing all duplicate values of the same country.
Select DISTINCT COUNTRY from MEDALS where MEDALTYPE = 'GOLD'.
The third expression is LIMIT, LIMIT is used for
restricting the number of rows retrieved from the database.
Example, retrieve just the first 10 rows in a table.
Select * from tablename LIMIT 10.
This can be very useful to examine the results set by looking at just a few
rows instead of retrieving the entire result set which may be very large.
Example, retrieve just a few rows in the MEDALS table for a particular year.
Select * from MEDALS where YEAR = 2018 LIMIT 5.
In this video we looked at some useful expressions that are used with select
statements, namely the COUNT, DISTINCT, and LIMIT built-in functions.
Thanks for watching this video.
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