- A non-recursive, or seed query class.
- A recursive query class.
- A UNION ALL set operator to connect the query components.
- A terminal condition to prevent infinite recursion.
A recursive view definition can contain multiple seed and recursive statements.
Normally, you should specify the terminal condition explicitly in the WHERE clause of the recursive statement, but such an explicit condition is not mandatory because recursive queries are implicitly limited by limits on user spool space and system disk space.
Consider the following basic BNF (BNF, or Backus-Naur Form, is a metalanguage used to define computer languages) for the CREATE RECURSIVE VIEW statement.
CREATE RECURSIVE VIEW <view_name> [(<column_name_list>)] AS ( <query_expression_1> UNION ALL <query_expression_2>
|Syntax element …||Specifies …|
|<view_name>||the name by which the view is referenced.|
|<column_name_list>||a set of base table column names and expressions from which the view referenced by <view_name> is to be constructed.|
|<query_expression_1>||a seed statement.|
|<query_expression_2>||a recursive statement.|
Angle bracket characters (<>) indicate an identifier or complex syntax element and straight bracket characters () indicate an optional syntax component.
The following example is based on the Flights table defined in The Concept of Recursion.
CREATE RECURSIVE VIEW reachable_from (source,destination,depth) AS ( SELECT root.source, root.destination, 0 AS depth FROM flights AS root WHERE root.source = 'Paris' UNION ALL SELECT in1.source, out1.destination, in1.depth + 1 FROM reachable_from AS in1, flights AS out1 WHERE in1.destination = out1.source AND in1.depth <= 100);
How does this example related to the BNF for CREATE RECURSIVE VIEW? The units of the BNF correspond to this example recursive view definition as follows.
|BNF element …||Corresponds to this example query element …|
|column_name_list||source, destination, depth|
SELECT root.source, root.destination, 0 AS depth FROM flights AS root WHERE root.source = 'Paris'
This is the seed statement (see Seed Statement Component of the View Definition) for the recursive view definition. The statement does not reference any recursive relations at any point in its definition.
SELECT in1.source,out1.destination,in1.depth + 1 FROM reachable_from AS in1, flights AS out1 WHERE in1.destination = out1.source AND in1.depth <=100
This is the recursive statement (see Recursive Statement Component of the View Definition) for the recursive view definition. The recursion occurs because the view name reachable_from is referenced in the FROM clause of this statement.