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<h1 class="topictitle1">Use recursive queries</h1>
<div><p>This topic introduces the use of recursive common table expressions
and recursive views.</p>
<div class="section"><p>Some applications work with data that is recursive in nature.
A Bill of Materials (BOM) application, for instance, works with the expansion
of parts and its component subparts. For example, a chair might be made of
a seat unit and a leg assembly. The seat unit might consist of a seat and
two arms. Each of these parts can be further broken down into its subparts
until there is a list of all the parts needed to build a chair. This type
of query can be handled by using a recursive common table expression or a
recursive view.</p>
</div>
<div class="section"><div class="p">In the following trip planner examples, airline flights and train
connections are used to find transportation paths between cities. The following
table definitions and data are used in the examples.<pre><strong>CREATE TABLE</strong> FLIGHTS (DEPARTURE <strong>CHAR</strong>(20),
ARRIVAL <strong>CHAR</strong>(20),
CARRIER <strong>CHAR</strong>(15),
FLIGHT_NUMBER <strong>CHAR</strong>(5),
PRICE <strong>INT</strong>)
<strong>INSERT INTO</strong> FLIGHTS <strong>VALUES</strong>('New York', 'Paris', 'Atlantic', '234', 400)
<strong>INSERT INTO</strong> FLIGHTS <strong>VALUES</strong>('Chicago', 'Miami', 'NA Air', '2334', 300)
<strong>INSERT INTO</strong> FLIGHTS <strong>VALUES</strong>('New York', 'London', 'Atlantic', '5473', 350)
<strong>INSERT INTO</strong> FLIGHTS <strong>VALUES</strong>('London', 'Athens' , 'Mediterranean', '247', 340)
<strong>INSERT INTO</strong> FLIGHTS <strong>VALUES</strong>('Athens', 'Nicosia' , 'Mediterranean', '2356', 280)
<strong>INSERT INTO</strong> FLIGHTS <strong>VALUES</strong>('Paris', 'Madrid' , 'Euro Air', '3256', 380)
<strong>INSERT INTO</strong> FLIGHTS <strong>VALUES</strong>('Paris', 'Cairo' , 'Euro Air', '63', 480)
<strong>INSERT INTO</strong> FLIGHTS <strong>VALUES</strong>('Chicago', 'Frankfurt', 'Atlantic', '37', 480)
<strong>INSERT INTO</strong> FLIGHTS <strong>VALUES</strong>('Frankfurt', 'Moscow', 'Asia Air', '2337', 580)
<strong>INSERT INTO</strong> FLIGHTS <strong>VALUES</strong>('Frankfurt', 'Beijing', 'Asia Air', '77', 480)
<strong>INSERT INTO</strong> FLIGHTS <strong>VALUES</strong>('Moscow', 'Tokyo', 'Asia Air', '437', 680)
<strong>INSERT INTO</strong> FLIGHTS <strong>VALUES</strong>('Frankfurt', 'Vienna', 'Euro Air', '59', 200)
<strong>INSERT INTO</strong> FLIGHTS <strong>VALUES</strong>('Paris', 'Rome', 'Euro Air', '534', 340)
<strong>INSERT INTO</strong> FLIGHTS <strong>VALUES</strong>('Miami', 'Lima', 'SA Air', '5234', 530)
<strong>INSERT INTO</strong> FLIGHTS <strong>VALUES</strong>('New York', 'Los Angeles', 'NA Air', '84', 330)
<strong>INSERT INTO</strong> FLIGHTS <strong>VALUES</strong>('Los Angeles', 'Tokyo', 'Pacific Air', '824', 530)
<strong>INSERT INTO</strong> FLIGHTS <strong>VALUES</strong>('Tokyo', 'Hong Kong', 'Asia Air', '94', 330)
<strong>INSERT INTO</strong> FLIGHTS <strong>VALUES</strong>('Washington', 'Toronto', 'NA Air', '104', 250)
<strong>CREATE TABLE</strong> TRAINS(DEPARTURE <strong>CHAR</strong>(20),
ARRIVAL <strong>CHAR</strong>(20),
RAILLINE <strong>CHAR</strong>(15),
TRAIN <strong>CHAR</strong>(5),
PRICE <strong>INT</strong>)
<strong>INSERT INTO</strong> TRAINS <strong>VALUES</strong>('Chicago', 'Washington', 'UsTrack', '323', 90)
<strong>INSERT INTO</strong> TRAINS <strong>VALUES</strong>('Madrid', 'Barcelona', 'EuroTrack', '5234', 60)
<strong>INSERT INTO</strong> TRAINS <strong>VALUES</strong>('Washington' , 'Boston' , 'UsTrack', '232', 50)</pre>
</div>
</div>
<div class="section"><div class="p">Now that the tables are set up, the data can be queried to find
information about the airline network. Suppose you want to find out what cities
you can fly to if you start in Chicago, and how many separate flights it will
take to get there. The following query shows you that information.<pre><strong>WITH</strong> destinations (origin, departure, arrival, flight_count) <strong>AS</strong>
(<strong>SELECT</strong> a.departure, a.departure, a.arrival, 1
<strong>FROM</strong> flights a
<strong>WHERE</strong> a.departure = 'Chicago'
<strong>UNION ALL</strong>
<strong>SELECT</strong> r.origin, b.departure, b.arrival, r.flight_count + 1
<strong>FROM</strong> destinations r, flights b
<strong>WHERE</strong> r.arrival = b.departure)
<strong>SELECT</strong> origin, departure, arrival, flight_count
<strong>FROM</strong> destinations</pre>
</div>
</div>
<div class="section"><p>This query returns the following information:</p>
<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" frame="border" border="1" rules="all"><caption>Table 1. Results
of the previous query</caption><thead align="left"><tr valign="bottom"><th valign="bottom" id="d0e240">ORIGIN</th>
<th valign="bottom" id="d0e242">DEPARTURE</th>
<th valign="bottom" id="d0e244">ARRIVAL</th>
<th valign="bottom" id="d0e246">FLIGHT_COUNT</th>
</tr>
</thead>
<tbody><tr><td valign="top" headers="d0e240 ">Chicago</td>
<td valign="top" headers="d0e242 ">Chicago</td>
<td valign="top" headers="d0e244 ">Miami</td>
<td valign="top" headers="d0e246 ">1</td>
</tr>
<tr><td valign="top" headers="d0e240 ">Chicago</td>
<td valign="top" headers="d0e242 ">Chicago</td>
<td valign="top" headers="d0e244 ">Frankfurt</td>
<td valign="top" headers="d0e246 ">1</td>
</tr>
<tr><td valign="top" headers="d0e240 ">Chicago</td>
<td valign="top" headers="d0e242 ">Miami</td>
<td valign="top" headers="d0e244 ">Lima</td>
<td valign="top" headers="d0e246 ">2</td>
</tr>
<tr><td valign="top" headers="d0e240 ">Chicago</td>
<td valign="top" headers="d0e242 ">Frankfurt</td>
<td valign="top" headers="d0e244 ">Moscow</td>
<td valign="top" headers="d0e246 ">2</td>
</tr>
<tr><td valign="top" headers="d0e240 ">Chicago</td>
<td valign="top" headers="d0e242 ">Frankfurt</td>
<td valign="top" headers="d0e244 ">Beijing</td>
<td valign="top" headers="d0e246 ">2</td>
</tr>
<tr><td valign="top" headers="d0e240 ">Chicago</td>
<td valign="top" headers="d0e242 ">Frankfurt</td>
<td valign="top" headers="d0e244 ">Vienna</td>
<td valign="top" headers="d0e246 ">2</td>
</tr>
<tr><td valign="top" headers="d0e240 ">Chicago</td>
<td valign="top" headers="d0e242 ">Moscow</td>
<td valign="top" headers="d0e244 ">Tokyo</td>
<td valign="top" headers="d0e246 ">3</td>
</tr>
<tr><td valign="top" headers="d0e240 ">Chicago</td>
<td valign="top" headers="d0e242 ">Tokyo</td>
<td valign="top" headers="d0e244 ">Hong Kong</td>
<td valign="top" headers="d0e246 ">4</td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="section"><p>This recursive query is written in two parts. The first part of
the common table expression is called the <em>intialization fullselect</em>.
It selects the first rows for the result set of the common table expression.
In this example, it selects the two rows in the <em>flights</em> table that
get you directly to another location from Chicago. It also initializes the
number of flight legs to one for each row it selects.</p>
<p>The second part
of the recursive query joins the rows from the current result set of the common
table expression with other rows from the original table. It is called the <em>iterative
fullselect</em>. This is where the recursion is introduced. Notice that the
rows that have already been selected for the result set are referenced by
using the name of the common table expression as the table name and the common
table expression result column names as the column names.</p>
<p>In this recursive
part of the query, any rows from the original table that you can get to from
each of the previously selected arrival cities are selected. A previously
selected row's arrival city becomes the new departure city. Each row from
this recursive select increments the flight count to the destination by one
more flight. As these new rows are added to the common table expression result
set, they are also fed into the iterative fullselect to generate more result
set rows. In the data for the final result, you can see that the total number
of flights is actually the total number of recursive joins (plus 1) it took
to get to that arrival city.</p>
<div class="p">A recursive view looks very similar to
a recursive common table expression. You can write the previous recursive
common table expression as a recursive view like this:<pre><strong>CREATE VIEW</strong> destinations (origin, departure, arrival, flight_count) <strong>AS</strong>
<strong>SELECT</strong> departure, departure, arrival, 1
<strong>FROM</strong> flights
<strong>WHERE</strong> departure = 'Chicago'
<strong>UNION ALL</strong>
<strong>SELECT</strong> r.origin, b.departure, b.arrival, r.flight_count + 1
<strong>FROM</strong> destinations r, flights b
<strong>WHERE</strong> r.arrival = b.departure)</pre>
</div>
<p>The interactive fullselect part of this view definition
refers to the view itself. Selection from this view returns the same rows
as you get from the previous recursive common table expression.</p>
</div>
<div class="section"><h4 class="sectiontitle">Example: Two starting cities</h4><p>Now, to make the query
a bit more complicated, suppose you are willing to fly from either Chicago
or New York, and you want to know where you could go and how much it would
cost.</p>
<pre><strong>WITH</strong> destinations (departure, arrival, connections, cost) <strong>AS</strong>
(<strong>SELECT</strong> a.departure, a.arrival, 0, price
<strong>FROM</strong> flights a
<strong>WHERE</strong> a.departure = 'Chicago' <strong>OR</strong>
a.departure = 'New York'
<strong>UNION ALL</strong>
<strong>SELECT</strong> r.departure, b.arrival, r.connections + 1,
r.cost + b.price
<strong>FROM</strong> destinations r, flights b
<strong>WHERE</strong> r.arrival = b.departure)
<strong>SELECT</strong> departure, arrival, connections, cost
<strong>FROM</strong> destinations</pre>
<p>This query returns the following
information:</p>
<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" frame="border" border="1" rules="all"><caption>Table 2. </caption><thead align="left"><tr valign="bottom"><th valign="bottom" id="d0e422">DEPARTURE</th>
<th valign="bottom" id="d0e424">ARRIVAL</th>
<th valign="bottom" id="d0e426">CONNECTIONS</th>
<th valign="bottom" id="d0e428"> COST</th>
</tr>
</thead>
<tbody><tr><td valign="top" headers="d0e422 ">Chicago </td>
<td valign="top" headers="d0e424 ">Miami</td>
<td valign="top" headers="d0e426 ">0</td>
<td valign="top" headers="d0e428 ">300</td>
</tr>
<tr><td valign="top" headers="d0e422 ">Chicago </td>
<td valign="top" headers="d0e424 ">Frankfurt</td>
<td valign="top" headers="d0e426 ">0</td>
<td valign="top" headers="d0e428 ">480</td>
</tr>
<tr><td valign="top" headers="d0e422 ">New York</td>
<td valign="top" headers="d0e424 ">Paris</td>
<td valign="top" headers="d0e426 ">0</td>
<td valign="top" headers="d0e428 ">400</td>
</tr>
<tr><td valign="top" headers="d0e422 ">New York</td>
<td valign="top" headers="d0e424 ">London</td>
<td valign="top" headers="d0e426 ">0</td>
<td valign="top" headers="d0e428 ">350</td>
</tr>
<tr><td valign="top" headers="d0e422 ">New York</td>
<td valign="top" headers="d0e424 ">Los Angeles</td>
<td valign="top" headers="d0e426 ">0</td>
<td valign="top" headers="d0e428 ">330</td>
</tr>
<tr><td valign="top" headers="d0e422 ">Chicago</td>
<td valign="top" headers="d0e424 ">Lima</td>
<td valign="top" headers="d0e426 ">1</td>
<td valign="top" headers="d0e428 ">830</td>
</tr>
<tr><td valign="top" headers="d0e422 ">Chicago</td>
<td valign="top" headers="d0e424 ">Moscow</td>
<td valign="top" headers="d0e426 ">1</td>
<td valign="top" headers="d0e428 ">1,060</td>
</tr>
<tr><td valign="top" headers="d0e422 ">Chicago</td>
<td valign="top" headers="d0e424 ">Beijing</td>
<td valign="top" headers="d0e426 ">1</td>
<td valign="top" headers="d0e428 ">960</td>
</tr>
<tr><td valign="top" headers="d0e422 ">Chicago</td>
<td valign="top" headers="d0e424 ">Vienna</td>
<td valign="top" headers="d0e426 ">1</td>
<td valign="top" headers="d0e428 ">680</td>
</tr>
<tr><td valign="top" headers="d0e422 ">New York</td>
<td valign="top" headers="d0e424 ">Madrid</td>
<td valign="top" headers="d0e426 ">1</td>
<td valign="top" headers="d0e428 ">780</td>
</tr>
<tr><td valign="top" headers="d0e422 ">New York</td>
<td valign="top" headers="d0e424 ">Cairo</td>
<td valign="top" headers="d0e426 ">1</td>
<td valign="top" headers="d0e428 ">880</td>
</tr>
<tr><td valign="top" headers="d0e422 ">New York</td>
<td valign="top" headers="d0e424 ">Rome</td>
<td valign="top" headers="d0e426 ">1</td>
<td valign="top" headers="d0e428 ">740</td>
</tr>
<tr><td valign="top" headers="d0e422 ">New York</td>
<td valign="top" headers="d0e424 ">Athens</td>
<td valign="top" headers="d0e426 ">1</td>
<td valign="top" headers="d0e428 ">690</td>
</tr>
<tr><td valign="top" headers="d0e422 ">New York</td>
<td valign="top" headers="d0e424 ">Tokyo</td>
<td valign="top" headers="d0e426 ">1</td>
<td valign="top" headers="d0e428 ">860</td>
</tr>
<tr><td valign="top" headers="d0e422 ">Chicago</td>
<td valign="top" headers="d0e424 ">Tokyo</td>
<td valign="top" headers="d0e426 ">2</td>
<td valign="top" headers="d0e428 ">1,740</td>
</tr>
<tr><td valign="top" headers="d0e422 ">New York</td>
<td valign="top" headers="d0e424 ">Nicosia</td>
<td valign="top" headers="d0e426 ">2</td>
<td valign="top" headers="d0e428 ">970</td>
</tr>
<tr><td valign="top" headers="d0e422 ">New York</td>
<td valign="top" headers="d0e424 ">Hong Kong</td>
<td valign="top" headers="d0e426 ">2</td>
<td valign="top" headers="d0e428 ">1,190</td>
</tr>
<tr><td valign="top" headers="d0e422 ">Chicago</td>
<td valign="top" headers="d0e424 ">Hong Kong</td>
<td valign="top" headers="d0e426 ">3</td>
<td valign="top" headers="d0e428 ">2,070</td>
</tr>
</tbody>
</table>
</div>
<p>For each returned row, the results show the starting departure
city and the final destination city. It counts the number of connections needed
rather than the total number of flight and adds up the total cost for all
the flights.</p>
</div>
<div class="section"><h4 class="sectiontitle">Example: Two tables used for recursion</h4><p>Now, suppose
you start in Chicago but add in transportation by railway in addition to the
airline flights, and you want to know which cities you can go to.</p>
<p>The
following query returns that information:</p>
<pre><strong>WITH</strong> destinations (departure, arrival, connections, flights, trains, cost) <strong>AS</strong>
(<strong>SELECT</strong> f.departure, f.arrival, 0, 1, 0, price
<strong>FROM</strong> flights f
<strong>WHERE</strong> f.departure = 'Chicago'
<strong>UNION ALL</strong>
<strong>SELECT</strong> t.departure, t.arrival, 0, 0, 1, price
<strong>FROM</strong> trains t
<strong>WHERE</strong> t.departure = 'Chicago'
<strong>UNION ALL</strong>
<strong>SELECT</strong> r.departure, b.arrival, r.connections + 1 , r.flights + 1, r.trains,
r.cost + b.price
<strong>FROM</strong> destinations r, flights b
<strong>WHERE</strong> r.arrival = b.departure
<strong>UNION ALL</strong>
<strong>SELECT</strong> r.departure, c.arrival, r.connections + 1 ,
r.flights, r.trains + 1, r.cost + c.price
<strong>FROM</strong> destinations r, trains c
<strong>WHERE</strong> r.arrival = c.departure)
<strong>SELECT</strong> departure, arrival, connections, flights, trains, cost
<strong>FROM</strong> destinations</pre>
<p>This query returns the following
information:</p>
<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" frame="border" border="1" rules="all"><caption>Table 3. Results of the previous query</caption><thead align="left"><tr valign="bottom"><th valign="bottom" id="d0e674">DEPARTURE</th>
<th valign="bottom" id="d0e676">ARRIVAL</th>
<th valign="bottom" id="d0e678">CONNECTIONS</th>
<th valign="bottom" id="d0e680">FLIGHTS</th>
<th valign="bottom" id="d0e682">TRAINS</th>
<th valign="bottom" id="d0e684">COST</th>
</tr>
</thead>
<tbody><tr><td valign="top" headers="d0e674 ">Chicago</td>
<td valign="top" headers="d0e676 ">Miami</td>
<td valign="top" headers="d0e678 ">0</td>
<td valign="top" headers="d0e680 ">1</td>
<td valign="top" headers="d0e682 ">0</td>
<td valign="top" headers="d0e684 ">300</td>
</tr>
<tr><td valign="top" headers="d0e674 ">Chicago</td>
<td valign="top" headers="d0e676 ">Frankfurt</td>
<td valign="top" headers="d0e678 ">0</td>
<td valign="top" headers="d0e680 ">1</td>
<td valign="top" headers="d0e682 ">0</td>
<td valign="top" headers="d0e684 ">480</td>
</tr>
<tr><td valign="top" headers="d0e674 ">Chicago</td>
<td valign="top" headers="d0e676 ">Washington</td>
<td valign="top" headers="d0e678 ">0</td>
<td valign="top" headers="d0e680 ">0</td>
<td valign="top" headers="d0e682 ">1</td>
<td valign="top" headers="d0e684 ">90</td>
</tr>
<tr><td valign="top" headers="d0e674 ">Chicago</td>
<td valign="top" headers="d0e676 ">Lima</td>
<td valign="top" headers="d0e678 ">1</td>
<td valign="top" headers="d0e680 ">2</td>
<td valign="top" headers="d0e682 ">0</td>
<td valign="top" headers="d0e684 ">830</td>
</tr>
<tr><td valign="top" headers="d0e674 ">Chicago</td>
<td valign="top" headers="d0e676 ">Moscow</td>
<td valign="top" headers="d0e678 ">1</td>
<td valign="top" headers="d0e680 ">2</td>
<td valign="top" headers="d0e682 ">0</td>
<td valign="top" headers="d0e684 ">1,060</td>
</tr>
<tr><td valign="top" headers="d0e674 ">Chicago</td>
<td valign="top" headers="d0e676 ">Beijing</td>
<td valign="top" headers="d0e678 ">1</td>
<td valign="top" headers="d0e680 ">2</td>
<td valign="top" headers="d0e682 ">0</td>
<td valign="top" headers="d0e684 ">960</td>
</tr>
<tr><td valign="top" headers="d0e674 ">Chicago</td>
<td valign="top" headers="d0e676 ">Vienna</td>
<td valign="top" headers="d0e678 ">1</td>
<td valign="top" headers="d0e680 ">2</td>
<td valign="top" headers="d0e682 ">0</td>
<td valign="top" headers="d0e684 ">680</td>
</tr>
<tr><td valign="top" headers="d0e674 ">Chicago</td>
<td valign="top" headers="d0e676 ">Toronto</td>
<td valign="top" headers="d0e678 ">1</td>
<td valign="top" headers="d0e680 ">1</td>
<td valign="top" headers="d0e682 ">1</td>
<td valign="top" headers="d0e684 ">340</td>
</tr>
<tr><td valign="top" headers="d0e674 ">Chicago</td>
<td valign="top" headers="d0e676 ">Boston</td>
<td valign="top" headers="d0e678 ">1</td>
<td valign="top" headers="d0e680 ">0</td>
<td valign="top" headers="d0e682 ">2</td>
<td valign="top" headers="d0e684 ">140</td>
</tr>
<tr><td valign="top" headers="d0e674 ">Chicago</td>
<td valign="top" headers="d0e676 ">Tokyo</td>
<td valign="top" headers="d0e678 ">2</td>
<td valign="top" headers="d0e680 ">3</td>
<td valign="top" headers="d0e682 ">0</td>
<td valign="top" headers="d0e684 ">1,740</td>
</tr>
<tr><td valign="top" headers="d0e674 ">Chicago</td>
<td valign="top" headers="d0e676 ">Hong Kong</td>
<td valign="top" headers="d0e678 ">3</td>
<td valign="top" headers="d0e680 ">4</td>
<td valign="top" headers="d0e682 ">0</td>
<td valign="top" headers="d0e684 ">2,070</td>
</tr>
</tbody>
</table>
</div>
<p>In this example, there are two parts of the common table expression
that provide initialization values to the query: one for flights and one for
trains. For each of the result rows, there are two recursive references to
get from the previous arrival location to the next possible destination: one
for continuing by air, the other for continuing by train. In the final results,
you would see how many connections are needed and how many airline or train
trips can be taken.</p>
</div>
<div class="section"><h4 class="sectiontitle">Example: DEPTH FIRST and BREADTH FIRST options</h4><p>The
two examples here show the difference in the result set row order based on
whether the recursion is processed depth first or breadth first.</p>
<div class="note"><span class="notetitle">Note:</span> The
search clause is not supported for recursive views. You can define a view
that contains a recursive common table expression to get this function.</div>
<p>The
option to determine the result using breadth first or depth first is a recursive
relationship sort based on the recursive join column specified for the SEARCH
BY clause. When the recursion is handled breadth first, all children are processed
first, then all grandchildren, then all great grandchildren. When the recursion
is handled depth first, the full recursive ancestry chain of one child is
processed before going to the next child.</p>
<p>In both of these cases, you
specify an extra column name that is used by the recursive process to keep
track of the depth first or breadth first ordering. This column must be used
in the ORDER BY clause of the outer query to get the rows back in the specified
order. If this column is not used in the ORDER BY, the DEPTH FIRST or BREADTH
FIRST processing option is ignored. </p>
<p>The selection of which column to
use for the SEARCH BY column is important. To have any meaning in the result,
it must be the column that is used in the iterative fullselect to join from
the initialization fullselect. In this example, ARRIVAL is the column to use.</p>
<p>The
following query returns that information:</p>
<pre><strong>WITH</strong> destinations (departure, arrival, connections, cost) <strong>AS</strong>
(<strong>SELECT</strong> f.departure, f.arrival, 0, price
<strong>FROM</strong> flights f
<strong>WHERE</strong> f.departure = 'Chicago'
<strong>UNION ALL</strong>
<strong>SELECT</strong> r.departure, b.arrival, r.connections + 1,
r.cost + b.price
<strong>FROM</strong> destinations r, flights b
<strong>WHERE</strong> r.arrival = b.departure)
<strong>SEARCH DEPTH FIRST BY</strong> arrival <strong>SET</strong> ordcol
<strong>SELECT</strong> *
<strong>FROM</strong> destinations
<strong>ORDER BY</strong> ordcol</pre>
<p>This query returns the following information:</p>
<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" frame="border" border="1" rules="all"><caption>Table 4. Results of the previous query</caption><thead align="left"><tr valign="bottom"><th valign="bottom" id="d0e902">DEPARTURE</th>
<th valign="bottom" id="d0e904">ARRIVAL</th>
<th valign="bottom" id="d0e906">CONNECTIONS</th>
<th valign="bottom" id="d0e908">COST</th>
</tr>
</thead>
<tbody><tr><td valign="top" headers="d0e902 ">Chicago</td>
<td valign="top" headers="d0e904 ">Miami</td>
<td valign="top" headers="d0e906 ">0</td>
<td valign="top" headers="d0e908 ">300</td>
</tr>
<tr><td valign="top" headers="d0e902 ">Chicago</td>
<td valign="top" headers="d0e904 ">Lima</td>
<td valign="top" headers="d0e906 ">1</td>
<td valign="top" headers="d0e908 ">830</td>
</tr>
<tr><td valign="top" headers="d0e902 ">Chicago</td>
<td valign="top" headers="d0e904 ">Frankfurt</td>
<td valign="top" headers="d0e906 ">0</td>
<td valign="top" headers="d0e908 ">480</td>
</tr>
<tr><td valign="top" headers="d0e902 ">Chicago</td>
<td valign="top" headers="d0e904 ">Moscow</td>
<td valign="top" headers="d0e906 ">1</td>
<td valign="top" headers="d0e908 ">1,060</td>
</tr>
<tr><td valign="top" headers="d0e902 ">Chicago</td>
<td valign="top" headers="d0e904 ">Tokyo</td>
<td valign="top" headers="d0e906 ">2</td>
<td valign="top" headers="d0e908 ">1,740</td>
</tr>
<tr><td valign="top" headers="d0e902 ">Chicago</td>
<td valign="top" headers="d0e904 ">Hong Kong</td>
<td valign="top" headers="d0e906 ">3</td>
<td valign="top" headers="d0e908 ">2,070</td>
</tr>
<tr><td valign="top" headers="d0e902 ">Chicago</td>
<td valign="top" headers="d0e904 ">Beijing</td>
<td valign="top" headers="d0e906 ">1</td>
<td valign="top" headers="d0e908 ">960</td>
</tr>
<tr><td valign="top" headers="d0e902 ">Chicago</td>
<td valign="top" headers="d0e904 ">Vienna</td>
<td valign="top" headers="d0e906 ">1</td>
<td valign="top" headers="d0e908 ">680</td>
</tr>
</tbody>
</table>
</div>
<p>In this result data, you can see that all destinations that are
generated from the Chicago-to-Miami row are listed before the destinations
from the Chicago-to-Frankfort row.</p>
<p>Next, you can run the same query
but request the result to be ordered breadth first.</p>
<pre><strong>WITH</strong> destinations (departure, arrival, connections, cost) <strong>AS</strong>
(<strong>SELECT</strong> f.departure, f.arrival, 0, price
<strong>FROM</strong> flights f
<strong>WHERE</strong> f.departure='Chicago'
<strong>UNION ALL</strong>
<strong>SELECT</strong> r.departure, b.arrival, r.connections + 1,
r.cost + b.price
<strong>FROM</strong> destinations r, flights b
<strong>WHERE</strong> r.arrival = b.departure)
<strong>SEARCH BREADTH FIRST BY</strong> arrival <strong>SET</strong> ordcol
<strong>SELECT</strong> *
<strong>FROM</strong> destinations
<strong>ORDER BY</strong> ordcol</pre>
<p>This query returns the following information:</p>
<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" frame="border" border="1" rules="all"><caption>Table 5. Results of the previous query</caption><thead align="left"><tr valign="bottom"><th valign="bottom" id="d0e1042">DEPARTURE</th>
<th valign="bottom" id="d0e1044">ARRIVAL</th>
<th valign="bottom" id="d0e1046">CONNECTIONS</th>
<th valign="bottom" id="d0e1048">COST</th>
</tr>
</thead>
<tbody><tr><td valign="top" headers="d0e1042 ">Chicago</td>
<td valign="top" headers="d0e1044 ">Miami</td>
<td valign="top" headers="d0e1046 ">0</td>
<td valign="top" headers="d0e1048 ">300</td>
</tr>
<tr><td valign="top" headers="d0e1042 ">Chicago</td>
<td valign="top" headers="d0e1044 ">Frankfurt</td>
<td valign="top" headers="d0e1046 ">0</td>
<td valign="top" headers="d0e1048 ">480</td>
</tr>
<tr><td valign="top" headers="d0e1042 ">Chicago</td>
<td valign="top" headers="d0e1044 ">Lima</td>
<td valign="top" headers="d0e1046 ">1</td>
<td valign="top" headers="d0e1048 ">830</td>
</tr>
<tr><td valign="top" headers="d0e1042 ">Chicago</td>
<td valign="top" headers="d0e1044 ">Moscow</td>
<td valign="top" headers="d0e1046 ">1</td>
<td valign="top" headers="d0e1048 ">1,060</td>
</tr>
<tr><td valign="top" headers="d0e1042 ">Chicago</td>
<td valign="top" headers="d0e1044 ">Beijing</td>
<td valign="top" headers="d0e1046 ">1</td>
<td valign="top" headers="d0e1048 ">960</td>
</tr>
<tr><td valign="top" headers="d0e1042 ">Chicago</td>
<td valign="top" headers="d0e1044 ">Vienna</td>
<td valign="top" headers="d0e1046 ">1</td>
<td valign="top" headers="d0e1048 ">680</td>
</tr>
<tr><td valign="top" headers="d0e1042 ">Chicago</td>
<td valign="top" headers="d0e1044 ">Tokyo</td>
<td valign="top" headers="d0e1046 ">2</td>
<td valign="top" headers="d0e1048 ">1,740</td>
</tr>
<tr><td valign="top" headers="d0e1042 ">Chicago</td>
<td valign="top" headers="d0e1044 ">Hong Kong</td>
<td valign="top" headers="d0e1046 ">3</td>
<td valign="top" headers="d0e1048 ">2,070</td>
</tr>
</tbody>
</table>
</div>
<p>In this result data, you can see that all the direct connections
from Chicago are listed before the connecting flights. The data is identical
to the results from the previous query, but in a breadth first order.</p>
</div>
<div class="section"><h4 class="sectiontitle">Example: Cyclic</h4><p>The key to any recursive process,
whether it is a recursive programming algorithm or querying recursive data,
is that the recursion must be finite. If not, you will get into a never ending
loop. The CYCLE option allows you to safeguard against cyclic data. Not only
will it terminate repeating cycles but it also allows you to optionally output
a cycle mark indicator that may lead you to find cyclic data. </p>
<div class="note"><span class="notetitle">Note:</span> The
cycle clause is not supported for recursive views. You can define a view that
contains a recursive common table expression to get this function.</div>
<p>For
a final example, suppose we have a cycle in the data. By adding one more row
to the table, there is now a flight from Cairo to Paris and one from Paris
to Cairo. Without accounting for possible cyclic data like this, it is quite
easy to generate a query that will go into an infinite loop processing the
data.</p>
<p>The following query returns that information:</p>
<pre><strong>INSERT INTO</strong> FLIGHTS <strong>VALUES</strong>('Cairo', 'Paris', 'Euro Air', '1134', 440)
<strong>WITH</strong> destinations (departure, arrival, connections, cost, itinerary) <strong>AS</strong>
(<strong>SELECT</strong> f.departure, f.arrival, 1, price,
<strong>CAST</strong>(f.departure <strong>CONCAT</strong> f.arrival <strong>AS VARCHAR</strong>(2000))
<strong>FROM</strong> flights f
<strong>WHERE</strong> f.departure = 'New York'
<strong>UNION ALL</strong>
<strong>SELECT</strong> r.departure, b.arrival, r.connections + 1 ,
r.cost + b.price, <strong>CAST</strong>(r.itinerary <strong>CONCAT</strong> b.arrival <strong>AS</strong> <strong>VARCHAR</strong>(2000))
<strong>FROM</strong> destinations r, flights b
<strong>WHERE</strong> r.arrival = b.departure)
<strong>CYCLE</strong> arrival <strong>SET</strong> cyclic_data <strong>TO</strong> '1' <strong>DEFAULT</strong> '0'
<strong>SELECT</strong> departure, arrival, itinerary, cyclic_data
<strong> FROM</strong> destinations
<strong>ORDER BY</strong> cyclic_data</pre>
<p>This query returns the following
information:</p>
<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" frame="border" border="1" rules="all"><caption>Table 6. Results of the previous
query</caption><thead align="left"><tr valign="bottom"><th valign="bottom" width="14.000000000000002%" id="d0e1224">DEPARTURE</th>
<th valign="bottom" width="14.000000000000002%" id="d0e1226">ARRIVAL</th>
<th valign="bottom" width="57.99999999999999%" id="d0e1228">ITINERARY</th>
<th valign="bottom" width="14.000000000000002%" id="d0e1230">CYCLIC_DATA</th>
</tr>
</thead>
<tbody><tr><td valign="top" width="14.000000000000002%" headers="d0e1224 ">New York</td>
<td valign="top" width="14.000000000000002%" headers="d0e1226 ">Paris</td>
<td valign="top" width="57.99999999999999%" headers="d0e1228 ">New York     Paris</td>
<td valign="top" width="14.000000000000002%" headers="d0e1230 ">0</td>
</tr>
<tr><td valign="top" width="14.000000000000002%" headers="d0e1224 ">New York</td>
<td valign="top" width="14.000000000000002%" headers="d0e1226 ">London</td>
<td valign="top" width="57.99999999999999%" headers="d0e1228 ">New York     London</td>
<td valign="top" width="14.000000000000002%" headers="d0e1230 ">0</td>
</tr>
<tr><td valign="top" width="14.000000000000002%" headers="d0e1224 ">New York</td>
<td valign="top" width="14.000000000000002%" headers="d0e1226 ">Los Angeles</td>
<td valign="top" width="57.99999999999999%" headers="d0e1228 ">New York     Los Angeles</td>
<td valign="top" width="14.000000000000002%" headers="d0e1230 ">0</td>
</tr>
<tr><td valign="top" width="14.000000000000002%" headers="d0e1224 ">New York</td>
<td valign="top" width="14.000000000000002%" headers="d0e1226 ">Madrid</td>
<td valign="top" width="57.99999999999999%" headers="d0e1228 ">New York     Paris     Madrid</td>
<td valign="top" width="14.000000000000002%" headers="d0e1230 ">0</td>
</tr>
<tr><td valign="top" width="14.000000000000002%" headers="d0e1224 ">New York</td>
<td valign="top" width="14.000000000000002%" headers="d0e1226 ">Cairo</td>
<td valign="top" width="57.99999999999999%" headers="d0e1228 ">New York     Paris     Cairo</td>
<td valign="top" width="14.000000000000002%" headers="d0e1230 ">0</td>
</tr>
<tr><td valign="top" width="14.000000000000002%" headers="d0e1224 ">New York</td>
<td valign="top" width="14.000000000000002%" headers="d0e1226 ">Rome</td>
<td valign="top" width="57.99999999999999%" headers="d0e1228 ">New York     Paris     Rome</td>
<td valign="top" width="14.000000000000002%" headers="d0e1230 ">0</td>
</tr>
<tr><td valign="top" width="14.000000000000002%" headers="d0e1224 ">New York</td>
<td valign="top" width="14.000000000000002%" headers="d0e1226 ">Athens</td>
<td valign="top" width="57.99999999999999%" headers="d0e1228 ">New York     London     Athens</td>
<td valign="top" width="14.000000000000002%" headers="d0e1230 ">0</td>
</tr>
<tr><td valign="top" width="14.000000000000002%" headers="d0e1224 ">New York</td>
<td valign="top" width="14.000000000000002%" headers="d0e1226 ">Tokyo</td>
<td valign="top" width="57.99999999999999%" headers="d0e1228 ">New York     Los Angeles     Tokyo</td>
<td valign="top" width="14.000000000000002%" headers="d0e1230 ">0</td>
</tr>
<tr><td valign="top" width="14.000000000000002%" headers="d0e1224 ">New York</td>
<td valign="top" width="14.000000000000002%" headers="d0e1226 ">Paris</td>
<td valign="top" width="57.99999999999999%" headers="d0e1228 ">New York     Paris     Cairo     Paris</td>
<td valign="top" width="14.000000000000002%" headers="d0e1230 ">1</td>
</tr>
<tr><td valign="top" width="14.000000000000002%" headers="d0e1224 ">New York</td>
<td valign="top" width="14.000000000000002%" headers="d0e1226 ">Nicosia</td>
<td valign="top" width="57.99999999999999%" headers="d0e1228 ">New York     London     Athens     Nicosia</td>
<td valign="top" width="14.000000000000002%" headers="d0e1230 ">0</td>
</tr>
<tr><td valign="top" width="14.000000000000002%" headers="d0e1224 ">New York</td>
<td valign="top" width="14.000000000000002%" headers="d0e1226 ">Hong Kong</td>
<td valign="top" width="57.99999999999999%" headers="d0e1228 ">New York     Los Angeles     Tokyo     Hong
Kong</td>
<td valign="top" width="14.000000000000002%" headers="d0e1230 ">0</td>
</tr>
</tbody>
</table>
</div>
<p>In this example, the ARRIVAL column is defined in the CYCLE clause
as the column to use for detecting a cycle in the data. When a cycle is found,
a special column, CYCLIC_DATA in this case, is set to the character value
of '1' for the cycling row in the result set. All other rows will contain
the default value of '0'. When a cycle on the ARRIVAL column is found, processing
will not proceed any further in the data so the infinite loop will not happen.
To see if your data actually has a cyclic reference, the CYCLIC_DATA column
can be referenced in the outer query.</p>
</div>
</div>
<div>
<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="rbafytexas.htm" title="Learn a variety of ways of tailoring your query to gather data using the SELECT statement. One way to do this is to use the SELECT statement in a program to retrieve a specific row (for example, the row for an employee). Furthermore, you can use clauses to gather data in a specific way.">Retrieve data using the SELECT statement</a></div>
</div>
</div>
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