Using Relational Databases and SQL

Steven EmoryDepartment of Computer Science

California State University, Los Angeles

Lecture 3:Joins Part I

Miscellany

Questions regarding Lab and Homework #1?

Very happy with Lab and Homework #1

Updated Wiki page

Moved Functions past midterm

Moved Subqueries and Set Operations before midterm

The Problem

Question: Display each title name along with the name of the artist that made it.

Hmmm... let's trySELECT Title, ArtistID from Titles;

Hmmm... doesn't work

We want artist names too

Artist names, however, are in another table

The Solutions

Use a subquery

SELECT Title, (SELECT ArtistName FROM Artists WHERE ArtistID=T.ArtistID) AS 'ArtistName'FROM Titles T;

Use a join

SELECT Title, ArtistNameFROM Titles NATURAL JOIN Artists;

What is a Join?

A join is a subset of the Cartesian Product between two tables

What is a Cartesian Product?

The Cartesian Product of tables A and B is the set of all possible concatenated rows whose first component comes from A and whose second component comes from B

Cartesian Product Example

Given these two tables, what is the Cartesian Product?

SELECT ArtistName FROM Artists;

SELECT Title FROM Titles;

Cartesian Product Result

Cartesian Product

SELECT ArtistName, TitleFROM Artists, Titles;

66 total rows

Is this the answer we want? No! There is too much information.

Cartesian Products can be quite large

Join Conditions

Since many records in a Cartesian Product are not meaningful, we can eliminate them using a join condition

In general, most of the time, we want to keep only matching records (i.e. only when two values of a common attribute between the two tables are equal)

Join Condition Example

For example:

-- Two tablesSELECT * FROM Artists;SELECT * FROM Titles;

-- Cartesian ProductSELECT *FROM Artists, Titles;

-- Join (Equi-join)SELECT *FROM Artists A, Titles TWHERE A.ArtistID=T.ArtistID;

Table Aliases

When joining tables with common attribute names, MySQL will get confused if you say:

SELECT *FROM Artists, TitlesWHERE ArtistID=ArtistID;

To solve this we can give each table an alias name (unlike column aliases, do not wrap you names in quotes):

SELECT *FROM Artists A, Titles TWHERE A.ArtistID=T.ArtistID;

Ways to Do a Cartesian ProductSeveral ways to do a Cartesian Product:

Cartesian Product (Form #1: Equi-Join Syntax)SELECT *FROM Titles, Artists;

Cartesian Product (Form #2: Cross Join Syntax)SELECT *FROM Titles CROSS JOIN Artists;

Cartesian Product (Form #3: Inner Join Syntax)SELECT *FROM Titles INNER JOIN Artists;

Cartesian Product (Form #4: Join On/Using Syntax)SELECT *FROM Titles JOIN Artists;

Cartesian Product Warnings

Do not do a Cartesian Product on more than two tables unless you really know what you’re doing!

-- Takes a long time!!!SELECT * FROM Artists, Titles, Tracks;

MySQL Join Types

Natural (this week)

Equi- (this week)

Inner (this week)

Outer (next week)

Left

Right

Cross (this week)

Natural Joins

A Natural Join joins two or tables, automatically determining the join condition.

The join condition attributes are only displayed once when using SELECT * with a natural join.

Natural Join Syntax

Two tables:

SELECT attribute_listFROM table1 NATURAL JOIN table2;

Multiple tables:

SELECT attribute_listFROM table1NATURAL JOIN table2NATURAL JOIN table3...

Natural Join Examples

Two tables:

SELECT *FROM Artists NATURAL JOIN Titles;

Three tables:

SELECT *FROM ArtistsNATURAL JOIN TitlesNATURAL JOIN Tracks;

Equi-Joins

An equi-join is a Cartesian Product with a join condition specified in the WHERE clause

The join condition attributes will be displayed multiple times when using SELECT * with an equi-join.

You must use table aliases in the join condition to differentiate join attributes.

Equi-Join Syntax

Two tables:

SELECT attribute_listFROM table1 alias1, table2 alias2WHERE alias1.attribute = alias2.attribute;

Multiple tables:

SELECT attribute_listFROM table1 alias1, table2 alias2, table3 alias3, ...WHERE alias1.attribute = alias2.attributeAND alias2.attribute = alias3.attributeAND ...;

Equi-Join Examples

Two tables:

SELECT *FROM Artists A, Titles TWHERE A.ArtistID = T.ArtistID;

Three tables:

SELECT *FROM Artists A, Titles T, Tracks KWHERE A.ArtistID = T.ArtistID ANDT.TitleID = K.TitleID;

Inner Joins

Exact same thing as an equi-join, just using a different syntax: the JOIN ON syntax

The INNER keyword is optional

Inner Join Syntax

Two tables:

SELECT attribute_listFROM table1 alias1 [INNER] JOIN table2 alias2ON alias1.attribute = alias2.attribute;

Multiple tables:

SELECT attribute_listFROM table1 alias1[INNER] JOIN table2 alias2 ON alias1.attribute = alias2.attribute[INNER] JOIN table3 alias3 ON alias2.attribute = alias3.attribute...

Inner Join Examples

Two tables:

SELECT *FROM Artists A INNER JOIN Titles TON A.ArtistID = T.ArtistID;

Three tables:

SELECT *FROM Artists AINNER JOIN Titles T ON A.ArtistID = T.ArtistID INNER JOIN Tracks K ON T.TitleID = K.TitleID;

Join Using

Equivalent to a natural join, with the exception that the attributes to be used in the join condition are not determined automatically

The user must specify one or more column attributes for the join condition

Join Using Syntax

Two tables:

SELECT attribute_listFROM table1 JOIN table2USING(attribute_name);

Multiple tables:

SELECT attribute_listFROM table1JOIN table2 USING(attribute_name1)JOIN table3 USING(attribute_name2)...

Join Using Examples

Two tables:

SELECT *FROM Artists JOIN TitlesUSING(ArtistID);

Three tables:

SELECT *FROM ArtistsJOIN Titles USING(ArtistID) JOIN Tracks USING(TitleID);

Cross Joins

A cross join computes the Cartesian Product

A cross join is logically equivalent to:

SELECT * FROM table1, table2, table3, ...;

In MySQL, a cross join is equivalent to an inner join, using the same syntax, but uses the CROSS keyword instead of INNER keyword

In standard SQL2003, a cross join is not equivalent to an inner join. A cross join in standard SQL2003 cannot use the JOIN ON syntax.

Cross Join Syntax

Two tables:

SELECT attribute_listFROM table1 CROSS JOIN table2;

Multiple tables:

SELECT attribute_listFROM table1CROSS JOIN table2CROSS JOIN table3...