2. Multi table selects
So, we mentioned that we can refer to
multiple tables during the course of our
selects
How do we do this?
What does that do for us?
3. Joins
We went over the natural join before
There are actually a billion types of joins you
can explicitly state in ANSI SQL
Okay, fine. There are 5 types.
There are a lot of good images on the net
about this, but I’m uncomfortable with their
licensing terms, so I’m going to re-draw them
for you
4. Joining on
Most joins require you to join “on” a particular
statement
The statements are the same kind that are used
in WHERE clauses
Table1.id = Table2.AddressBookId
The PostgreSQL refers to this as the join
condition
5. Join types
INNER JOIN
FULL OUTER JOIN
LEFT OUTER JOIN
RIGHT OUTER JOIN
CROSS JOIN
6. Inner join
This is one of the more frequent types of joins
Finds all rows which meet the join condition
Example
SELECT * FROM AddressBook a INNER JOIN
CallList c on a.id = c.addressBookId;
a and c are aliases
8. Left outer join
Contains all records between T1 and T2 that
meet the join condition, and then any records
in T1 that don’t meet the join condition
Rows for which the join condition is not met, the
columns of T2 are padded with nulls
10. Right outer join
Contains all records between T1 and T2 that
meet the join condition, and then any records
in T2 that don’t meet the join condition
Rows for which the join condition is not met, the
columns of T1 are padded with nulls
12. Full outer join
Contains all records between T1 and T2 that
meet the join condition, and the combination
of a left outer join and right outer join are
appended onto the results
14. Cross join
Cross joins have no join condition
Cross joins literally return the Cartesian product
(discussed in the relational algebra slides)
15. Implicit join notation vs. explicit
Inner joins can be accomplished with “implicit”
join notation
In other words, we don’t literally use the terms
“inner join” in our select statement
We can use where clauses/multi table selects to
accomplish the same thing
16. Explicit join
SELECT * FROM AddressBook a INNER JOIN
CallList c on a.id = c.addressBookId;
18. Which to use?
Which should you use?
It’s up to you, really
My experience
I never really use explicit syntax
Implicit makes much more sense to me,
personally
19. Outer joins, cross joins in the real
world?
You can very much create a fully functional
database without using left outer joins, right
outer joins, etc.
These types of joins allow you to collate data in
ways that would be require much more data
munging
More queries, more tables
Truth be told, I didn’t realize their power until
writing these slides
20. Examples
Get a full list of employees and determine
who has not enrolled in benefits
SELECT * FROM
employees LEFT OUTER JOIN
benefitsEnrollment ON employees.id =
benefitsEnrollment.employeeId;
If an employee hasn’t enrolled in benefits, the
benefitsEnrollment columns would come back
as NULL
21. Views
A view is kind of like a virtual table
Views are defined as queries (SELECT
statements)
They can be queried like tables
They are read only
Syntax:
CREATE VIEW <view-name>
AS <query>
22. View example
CREATE VIEW phoneNumbers
AS SELECT phoneNumber FROM
AddressBook;
This would create a view that only contained
phone numbers from our AddressBook
23. More complex views
CREATE VIEW callsAndInfo AS
SELECT * FROM AddressBook a INNER JOIN
CallList c on a.id = c.addressBookId;
Views are more useful for more complex
queries
Can join these views on other tables as well
24. View danger!
Depending too heavily on views can cause
performance issues if you’re not careful
You can apply constraints to view queries which
have a heavy performance impact
If you have views that depend on views that
depend on views
25. Materialized views
PostgreSQL has materialized views, which
are views that are stored on disk
Periodically updated and stored
Regular views are not stored, so they always hit
dependencies in real time
Syntax:
CREATE MATERIALIZED VIEW <view-name>
AS <query>
26. Subqueries
You can use selects in your selects!
This is SUPER confusing
You can use them in both the FROM and the
WHERE
Used in the from, kind of like in-lining a view
Can use them in the where to grab values from
other tables
27. Subquery
SELECT * FROM
Stalkers s,
(SELECT * FROM AddressBook a INNER
JOIN CallList c on a.id = c.addressBookId) c
WHERE s.contactName = c.contactName;
28. Using subqueries in a from
SELECT * FROM UserProfile
WHERE userId = (SELECT userId FROM
MostPopularUser ORDER BY popularity
LIMIT 1);
