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4. The USING clause may be used instead of the ON clause for
join conditions based on equal values of columnsthat have the same
name in the source tables. The query result in this case shows the
join column/s only once.When the ON clause is used, the common
columns in the join condition are treated as distinct columns.
These
alternatives for join queries are illustrated below. Example
queries are obtained from the SAKILAdatabase,
which contains common attributes across tables.
Observe the appearance of the join column country_idtwice (i.e.,
the common columns from bothsource tables are displayed).
SELECT * FROM country JOIN city
ON country.country_id = city.country_id;
Selecting the common column in the SELECT clause requires
qualification via the table name.SELECT country.country_id,
country, city FROM country JOIN city
ON country.country_id = city.country_id;
Below are the equivalent queries with the USING clause. The join
column country_idappears onlyonce and does not have to be qualified
by a table name when included in the SELECT clause.
SELECT * FROM country JOIN city USING (country_id);
SELECT country_id, country, city FROM country JOIN city
USING (country_id);
5. TheNATURAL JOIN syntax may also be used instead of the USING
clause. This automatically joins tablesbased on equal values of the
combination of all columns that have identical names. This
eliminates the need for
specifying the join columns in the statement. Join columns
appear only once, just as in the USING clause.
Below are the correspondingNATURAL JOIN queries for the previous
exampies.SELECT * FROM country NATURAL JOIN city;
SELECT country_id, country, city FROM country NATURAL JOIN
city;
6. Qualifying column names with table names can be very time
consuming, particularly if table names are lengthy.Table aliases
can be used instead of table names.
SELECT a.country_id, country, city FROM country a JOIN city
b
ON a.country_id = b.country_id;
7. A join may involve the same table, that is, a table is joined
to itself (a.k.a. self-join). The following equivalentqueries
display the list of cities that belong to the same country as a
given city (sample given city is Lapu-Lapu).
SELECT b.city FROM city a JOIN city b
ON a.country_id = b.country_id WHERE a.city = "Lapu-Lapu";
SELECT b.city from city a JOIN city b
USING (country_id) WHERE a.city = "Lapu-Lapu";
8. Below are equivalent queries that illustrate a three-way join
(i.e., data is extracted from three tables). Ingeneral, to join
ntables, at least n-1join conditions/join columns are required.
SELECT * FROM film f JOIN film_actor faON f.film_id =
fa.film_id
JOIN actor a ON fa.actor_id = a.actor_id;
SELECT * FROM film JOIN film_actor USING (film_id)
JOIN actor USING (actor_id);
SELECT * FROM film NATURAL JOIN film_actor NATURAL JOIN
actor;
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III.Outer Joins1. The LEFT [OUTER] JOIN returns all rows from
the first source table regardless of whether the rows satisfythe
specified join condition or not. In effect, it returns not only the
result of the inner join, but also the rowsfrom the first source
table that do not have a match (via the join condition) in the
second source table. Theunmatched columns are displayed with null
values. The RIGHT [OUTER] JOIN is analogous to the left
join and returns all rows from the second source table
instead.
The following equivalent queries display film information along
with the corresponding actor IDs of theactors who are included in
the films. Based on the definition of the inner join, rows from
both tables arecombined together based on equal values of the
respective film_idcolumns from both source tables.
SELECT * FROM film JOIN film_actor USING (film_id)
ORDER BY actor_id;
SELECT * FROM film INNER JOIN film_actorON film.film_id =
film_actor.film_id
ORDER BY actor_id;
SELECT * FROM film NATURAL JOIN film_actor
ORDER BY actor_id;
The following equivalent queries display ALL films along with
the corresponding actor IDs of the actors whoare included in the
films, if there are any. Observe that the result has 3 more rows
than the previousquery. The first three rows contain information on
films that do not have corresponding actors.(film_actor columns are
null).
SELECT * FROM film LEFT JOIN film_actor USING (film_id)
ORDER BY actor_id;
SELECT * FROM film LEFT JOIN film_actor
ON film.film_id = film_actor.film_id
ORDER BY actor_id;
SELECT * FROM film NATURAL LEFT JOIN film_actor
ORDER BY actor_id;
The following equivalent queries display only those films
without actors. The outer join provides aconvenient way to
accomplish this.
SELECT * FROM film LEFT JOIN film_actor USING (film_id)
WHERE actor_id IS NULL;
SELECT * FROM film LEFT JOIN film_actor
ON film.film_id = film_actor.film_id WHERE actor_id IS
NULL;SELECT * FROM film NATURAL LEFT JOIN film_actor
WHERE actor_id IS NULL;
Equivalent queries can be created via the RIGHT JOIN provided
that the order of the source tables isreversed. Like the inner
join, the outer join can be extended to extract data from more than
two tables.
