Multi-table queries allow data from different tables to be combined by comparing common values between tables. The main types of multi-table queries are joins, unions, and subqueries. Joins combine columns from multiple tables and the most common type is the inner join, which returns rows where the compared values match. Outer joins return all rows from one table along with any matching rows from another table. Cartesian joins match each row from one table with every row from another table.

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