Introduction to Joins in SQL

1. Joins
CIS-182

2. Multi-Table Queries
• Multi-table queries allow data from
different tables to be combined and
compared
• Typically done by comparing a common
value
– Comparison done between like data types
– May be primary key/foreign key fields but
doesn’t have to be

3. Types of Multi-table Queries
• Joins: combine columns from multiple tables
– Combine publishers and titles to show title and
publisher name
• Unions: combine rows from different lists
– Create a list of editors and authors who live in
California
• Subqueries: use the results of one query to
determine result of a second query
– Find all books that have a higher than average
price

4. Joins
• Inner Join: create a row for each match of
values from two tables
• Outer Join: include all rows from one table
and any related data found in a second
table
• Cartesian Join: match each row in a table
with every row in a second table
– Also referred to as a cross join

5. Inner Joins
• Most common kind of join
• Get related data from two tables where
values match
– Also referred to as an equi-join
• May be done using FROM clause or
WHERE clause
– Microsoft does not fully support WHERE
clause in SQL Server 2008

6. Join With FROM
• Use the keyword JOIN with or without
INNER preceeding it
• Specify the fields to compare using ON
• Field names that are not unique must be
qualified by the table they’re in

7. From Clause Examples
SELECT *
FROM Publishers JOIN Titles
ON Publishers.pub_id = Titles.pub_id
SELECT *
FROM Publishers INNER JOIN Titles
ON Publishers.pub_id = Titles.pub_id

8. Join With WHERE
• List tables in FROM clause, separated by
comma
• WHERE clause defines the fields to
compare

9. Where Clause Example
SELECT *
FROM Publishers, Titles
WHERE Publishers.pub_id =
Titles.pub_id

10. Outer Joins
• Used when rows from one table should be
part of the result even if there are no
related rows in a second table
– Create a list of all publishers, including any
titles the publisher has published
• Direction must be specified
– Left/Right specify which table has the rows
which should always be included
– Full specifies that rows from both tables
should be included even if no match

11. Outer Join Example
• Include all publishers, even if they haven’t
published a book:
SELECT *
FROM Publishers LEFT OUTER JOIN
Titles ON Publishers.pub_id =
Titles.pub_id

12. Cartesian Join
• Match each row from table 1 with every row
from table 2
– Result is (table 1 row count)*(table 2 row count)
• May be done using CROSS JOIN
SELECT *
FROM Publishers, Titles
SELECT *
FROM Publishers CROSS JOIN Titles