Chapter 7 - Joining Tables
 No
Explicit Join in SQL
 Joins are implicitly expressed in the
WHERE clause
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Two Tables at a Time
 Can
Join Only Two Tables at a Time
 Use 'Connecting' Columns
– datatypes must match
– or convert correctly
 More
than one Join can be specified in a
Single SELECT statement
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Join Syntax
 SELECT select_list
FROM table_1, table_2
WHERE [table_1.]column
join_operator
[table_2.]column
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Example
 SELECT title,
pub_name
FROM titles, publishers
WHERE titles.pub_id = publishers.pub_id;
 Qualify
the column names with the table
names with there is ambiguity
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Aliases
 Remember
you can use short names as
aliases for the table names
 SELECT title,
pub_name
FROM titles t, publishers p
WHERE t.pub_id = p.pub_id;
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Example
 Find
the editors of the Secrets of Silicon
Valley
 SELECT ed_lname,
ed_fname, ed_pos
FROM editors e, titleditors te, titles t
WHERE e.ed_id = te.ed_id
AND t.title_id = te.title_id
AND title = 'Secrets of Silicon Valley';
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Unplanned Joins
 Joins
may be planned
– Primary Keys
– Foreign Keys
 Joins
do not have to be planned
– e.g. you may join the "city" column on the
authors and editors table
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Join Operators
 You
can use any of the relational operators
to express the relationship between the Join
columns
 Equality most often used
 SELECT title, pub_name
FROM titles, publishers
WHERE pub_name = ‘New Age Books’
AND title.pub_id = publishers.pub_id;
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Column Compatibility
 Join
columns need not have the same name
 They should be of the same datatype (or
convert to the same datatype)
 The join should be meaningful
 Nulls found in connecting columns will
never join.
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Data Replication
 Rows
are returned for every successful join
 Returned rows may replicate data for
display. (This does not change the data.)
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Connecting Columns
 You
don’t have to put connecting columns
in the select list
 You do need to qualify connecting columns
that have the same name in the select list.
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Example
 SELECT ed_name,
editors.city
FROM editors, publishers
WHERE editors.city = publishers.city
AND pub_name = ‘Algodata Infosystems’;
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Exercise
 Show
a list of the authors whose last name
is Ringer and the city they live in, but only
if they have written any books. Put in last
name order.
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Answer
 SELECT au_lname,
au_fname, city
FROM authors a, titleauthors ta
WHERE a.au_id = ta.au_id;
 If
no join is made from an author (au_id) to
the titleauthors table then the author has not
written any books.
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Join Operators
>
 >=
<
 <=
<
>
 *=
 =*
greater than
greater than or equal to
less than
less than or equal to
not equal to
Left Outer Join
Right Outer Join
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Example
 SELECT DISTINCT
s.sonum,
s.stor_id,
s.sdate,
sd.date_shipped
FROM sales s, salesdetails sd
WHERE s.sale_date < sd.date_shipped
AND s.sonum = sd.sonum;
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Joining More than Two Tables
 We
can only join two tables at a time…
… but we can do more than one ‘two-table’
join in a Select statement.
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Exercise
 Show
a list of books and the authors
who’ve written them. Use aliases.
– Remember: Many-to-Many relationships are
modeled with an association table
– Need to join three tables to get information
from both associated tables.
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Solution
 SELECT title,
au_lname, au_fname
FROM titles t, authors a, titleauthors ta
WHERE t.title_id = ta.title_id
AND a.au_id = ta.au_id
ORDER BY 1, 2;
 Notice
that the Primary keys are joined, but
not displayed in the Select list.
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Another Exercise
 Show
a list of books (by book title) and the
number of authors that have written each
book, if more than one author wrote the
book.
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Joins that don’t match
 Joins
only return the rows that satisfy all
the conditions in the WHERE clause…
…Unless you use an “Outer Join”
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Outer Joins
 Outer
Joins…
– display all the rows from one table (that meet
the WHERE clause criteria)
– display information from the other table only if
it matches the criteria, otherwise it displays
NULL
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Left vs. Right Outer Joins
 A left
outer join displays all the criteria
matching rows from the first named table
and the matching information only from the
second named table.
 A right outer join displays all the criteria
matching rows from the second named
table and the matching information only
from the first named table.
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Example - Left Outer Join
 SELECT au_fname,
au_lname, pub_name
FROM authors, publishers
WHERE authors.city *= publishers.city;
 This
displays all the rows from the authors
table and displays the pub_name in the
results set where the authors city is equal to
a city that the publisher is headquatered in.
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Example - Right Outer Join
 SELECT au_fname,
au_lname, pub_name
FROM authors, publishers
WHERE authors.city =* publishers.city;
 This
displays all the rows from the
publishers table and displays the authors
name columns in the results set where the
publishers city is equal to a city that the
author resides in.
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Exercise
 Part
1 - Show a list of the authors who are
also editors. Display their last name and
editor position.
 Part
2 - Show a list of all the authors.
Display their last name and display their
editor position if they are also an editor.
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Answer
 Part
1SELECT au_lname, au_fname, ed_pos
FROM authors a, editors e
WHERE au_id = ed_id;
 Part 2SELECT au_lname, au_fname, ed_pos
FROM authors a, editors e
WHERE au_id *= ed_id;
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Last Slide - Chapter 7
 Assignment
#5 due next week.
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