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><H1
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><A
NAME="TUTORIAL-JOIN"
>2.6. Joins Between Tables</A
></H1
><P
> Thus far, our queries have only accessed one table at a time.
Queries can access multiple tables at once, or access the same
table in such a way that multiple rows of the table are being
processed at the same time. A query that accesses multiple rows
of the same or different tables at one time is called a
<I
CLASS="FIRSTTERM"
>join</I
> query. As an example, say you wish to
list all the weather records together with the location of the
associated city. To do that, we need to compare the <TT
CLASS="STRUCTFIELD"
>city</TT
>
column of each row of the <TT
CLASS="STRUCTNAME"
>weather</TT
> table with the
<TT
CLASS="STRUCTFIELD"
>name</TT
> column of all rows in the <TT
CLASS="STRUCTNAME"
>cities</TT
>
table, and select the pairs of rows where these values match.
</P><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> This is only a conceptual model. The join is usually performed
in a more efficient manner than actually comparing each possible
pair of rows, but this is invisible to the user.
</P
></BLOCKQUOTE
></DIV
><P>
This would be accomplished by the following query:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT *
FROM weather, cities
WHERE city = name;</PRE
><P>
</P><PRE
CLASS="SCREEN"
> city | temp_lo | temp_hi | prcp | date | name | location
---------------+---------+---------+------+------------+---------------+-----------
San Francisco | 46 | 50 | 0.25 | 1994-11-27 | San Francisco | (-194,53)
San Francisco | 43 | 57 | 0 | 1994-11-29 | San Francisco | (-194,53)
(2 rows)</PRE
><P>
</P
><P
> Observe two things about the result set:
<P
></P
></P><UL
><LI
><P
> There is no result row for the city of Hayward. This is
because there is no matching entry in the
<TT
CLASS="STRUCTNAME"
>cities</TT
> table for Hayward, so the join
ignores the unmatched rows in the <TT
CLASS="STRUCTNAME"
>weather</TT
> table. We will see
shortly how this can be fixed.
</P
></LI
><LI
><P
> There are two columns containing the city name. This is
correct because the lists of columns from the
<TT
CLASS="STRUCTNAME"
>weather</TT
> and
<TT
CLASS="STRUCTNAME"
>cities</TT
> tables are concatenated. In
practice this is undesirable, though, so you will probably want
to list the output columns explicitly rather than using
<TT
CLASS="LITERAL"
>*</TT
>:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT city, temp_lo, temp_hi, prcp, date, location
FROM weather, cities
WHERE city = name;</PRE
><P>
</P
></LI
></UL
><P>
</P
><DIV
CLASS="FORMALPARA"
><P
><B
>Exercise: </B
> Attempt to determine the semantics of this query when the
<TT
CLASS="LITERAL"
>WHERE</TT
> clause is omitted.
</P
></DIV
><P
> Since the columns all had different names, the parser
automatically found which table they belong to. If there
were duplicate column names in the two tables you'd need to
<I
CLASS="FIRSTTERM"
>qualify</I
> the column names to show which one you
meant, as in:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT weather.city, weather.temp_lo, weather.temp_hi,
weather.prcp, weather.date, cities.location
FROM weather, cities
WHERE cities.name = weather.city;</PRE
><P>
It is widely considered good style to qualify all column names
in a join query, so that the query won't fail if a duplicate
column name is later added to one of the tables.
</P
><P
> Join queries of the kind seen thus far can also be written in this
alternative form:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT *
FROM weather INNER JOIN cities ON (weather.city = cities.name);</PRE
><P>
This syntax is not as commonly used as the one above, but we show
it here to help you understand the following topics.
</P
><P
>
Now we will figure out how we can get the Hayward records back in.
What we want the query to do is to scan the
<TT
CLASS="STRUCTNAME"
>weather</TT
> table and for each row to find the
matching <TT
CLASS="STRUCTNAME"
>cities</TT
> row(s). If no matching row is
found we want some <SPAN
CLASS="QUOTE"
>"empty values"</SPAN
> to be substituted
for the <TT
CLASS="STRUCTNAME"
>cities</TT
> table's columns. This kind
of query is called an <I
CLASS="FIRSTTERM"
>outer join</I
>. (The
joins we have seen so far are inner joins.) The command looks
like this:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT *
FROM weather LEFT OUTER JOIN cities ON (weather.city = cities.name);
city | temp_lo | temp_hi | prcp | date | name | location
---------------+---------+---------+------+------------+---------------+-----------
Hayward | 37 | 54 | | 1994-11-29 | |
San Francisco | 46 | 50 | 0.25 | 1994-11-27 | San Francisco | (-194,53)
San Francisco | 43 | 57 | 0 | 1994-11-29 | San Francisco | (-194,53)
(3 rows)</PRE
><P>
This query is called a <I
CLASS="FIRSTTERM"
>left outer
join</I
> because the table mentioned on the left of the
join operator will have each of its rows in the output at least
once, whereas the table on the right will only have those rows
output that match some row of the left table. When outputting a
left-table row for which there is no right-table match, empty (null)
values are substituted for the right-table columns.
</P
><DIV
CLASS="FORMALPARA"
><P
><B
>Exercise: </B
> There are also right outer joins and full outer joins. Try to
find out what those do.
</P
></DIV
><P
>
We can also join a table against itself. This is called a
<I
CLASS="FIRSTTERM"
>self join</I
>. As an example, suppose we wish
to find all the weather records that are in the temperature range
of other weather records. So we need to compare the
<TT
CLASS="STRUCTFIELD"
>temp_lo</TT
> and <TT
CLASS="STRUCTFIELD"
>temp_hi</TT
> columns of
each <TT
CLASS="STRUCTNAME"
>weather</TT
> row to the
<TT
CLASS="STRUCTFIELD"
>temp_lo</TT
> and
<TT
CLASS="STRUCTFIELD"
>temp_hi</TT
> columns of all other
<TT
CLASS="STRUCTNAME"
>weather</TT
> rows. We can do this with the
following query:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT W1.city, W1.temp_lo AS low, W1.temp_hi AS high,
W2.city, W2.temp_lo AS low, W2.temp_hi AS high
FROM weather W1, weather W2
WHERE W1.temp_lo < W2.temp_lo
AND W1.temp_hi > W2.temp_hi;
city | low | high | city | low | high
---------------+-----+------+---------------+-----+------
San Francisco | 43 | 57 | San Francisco | 46 | 50
Hayward | 37 | 54 | San Francisco | 46 | 50
(2 rows)</PRE
><P>
Here we have relabeled the weather table as <TT
CLASS="LITERAL"
>W1</TT
> and
<TT
CLASS="LITERAL"
>W2</TT
> to be able to distinguish the left and right side
of the join. You can also use these kinds of aliases in other
queries to save some typing, e.g.:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT *
FROM weather w, cities c
WHERE w.city = c.name;</PRE
><P>
You will encounter this style of abbreviating quite frequently.
</P
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