For today we will be practicing inserting and querying data using SQL.
Here is a website that will let us write queries to interact with some data. http://jxs.me/chinook-web/
On the left are the Tables with their fields. The right is where we will be writing our queries. The bottom is where we will see our results.
When creating tables we can specify a column as having a foreign key.
The datatype of our column must match the datatype of the column we are linking to (Usually a primary key which is usually an integer).
Create a new table called movies with an id, title and mediaTypeId. Make mediaTypeId a foreign key to the MediaType table on the MediaTypeId column.
CREATE TABLE movies (
id INTEGER PRIMARY KEY,
title TEXT,
mediaTypeId INTEGER,
FOREIGN KEY(mediaTypeId) REFERENCES MediaType(MediaTypeId)
);
Test this by adding a movie with a title and a mediaTypeId.
insert into movies (title, mediaTypeId) values ("Aladdin", 3)
Now query the movies table using a simple select statement and you should see your movie.
We can also add a foreign key to an existing table. Let's add one to our movies table that references genre on genreId
ALTER TABLE movies ADD COLUMN genreId INTEGER REFERENCES Genre(GenreId);
Query your movies table and you will see that the same row is now returned with an extra column.
We don't want to leave genre null so let's add a value using the update command. With an update command you always want to use a where clause (if you don't you will overwrite data on all records).
update movies set genreId=22 where id=1
Now that we know how to make foreign keys and change data let's do some practice queries. The simplest way to use a foreign key is via a join statement.
Let's join the artist and album tables so we can list the artist name next to each album name.
select a.title, ar.Name from album a join artist ar on a.ArtistId=ar.ArtistId
The next way to use a primary key is with a nested query/sub-select statement. By using parenthesis we can do a select inside of a select. This is really effective when you have a foreign key link between two tables because now we can filter our main query by criteria on a referenced table.
select * from Track where GenreId in (select GenreId from Genre where Name='Jazz' OR Name='Blues')
We just got all data from Track where the track was pointing to a record in the genre table that happend to be either Jazz or Blues.
The subquery (select GenreId from Genre where Name='Jazz' OR Name='Blues') returns an array of GenreId.
Our parent query then checks for Tracks where the Track Genre is in that array where GenreId in
Sometimes we want to remove values. Try this then select on the Employee table and look at the first Employee
update Employee set Phone=null where EmployeeId=1
Sometimes you want to know when there is no value. What if wanted to know all customers that purchased for themselves and not a company.
If you try this what do you see?
select * from customer where company = null
This, this works:
select * from customer where company is null
How many albums does each artist have? We could count manually, but no! Let's use a group by to do some aggregate counts for us.
Select all artist ids, artist names, and a count of how many albums they have.
select ar.artistId, ar.name, count(*) from artist ar join album a on ar.ArtistId=a.ArtistId group by ar.artistId
Order this by album count, largest first
We have hundreds of customers and want to know which countries our customers come from, but no duplicates!
select distinct country from customer
Always do a select before a delete to make sure you get back exactly what you want and only what you want to delete!
select * from customer where fax is not null
Then replace the select * with a delete and you're safely deleting rows.
delete from customer where fax is not null
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