# 15.00 - All Recursive View Definitions Must Be Linear - Teradata Database

## Teradata Database SQL Data Definition Language Detailed Topics

Product
Release Number
15.00
Content Type
Programming Reference
Publication ID
B035-1184-015K
Language
English (United States)

### All Recursive View Definitions Must Be Linear

SQL does not support non‑linear recursion.

When recursion is linear, invoking a recursive view produces at most 1 direct invocation of that recursive view. To enforce linearity, SQL restricts table references to a single occurrence in the recursive view definition, which can be specified either in the FROM clause of the recursive statement or in a subquery in the recursive statement, but not in both.

A recursion is said to be linear if it obeys the following rules.

• For every joined table in the recursive view definition, the recursively‑defined table is referenced only once.
• The recursively‑defined table is not referenced in both the FROM clause and in a subquery of the same query specification.
• The recursively‑defined table is not referenced more than once in the FROM clause of the query specification.
• If any of these conditions is not met, then the recursion is non‑linear.

The following create text is an example of a non‑valid attempt to create a recursive view definition. The definition is not valid because the recursive view Fibonacci is referenced more than once (using the column alias names p and pp, respectively) in the FROM clause of the recursively‑defined relation in the view definition. As a result, it violates the rule stated in the third bullet.

`     CREATE RECURSIVE VIEW fibonacci (n, f, mycount) AS`
`       SELECT a, b, 0 AS mycount`
`       FROM t`
`       WHERE (a=0`
`       AND    b=0)`
`       OR    (a=1`
`       AND    b=1)`
`     UNION ALL`
`       SELECT n+1, p.f + pp.f, p.mycount + 1`
`       FROM fibonacci AS p, fibonacci AS pp`
`       WHERE (p.n - 1) = pp.n`
`       AND   p.mycount <= 100;`

The following similarly conceived query that uses derived tables has the same problem.

`     CREATE RECURSIVE VIEW fibonacci (n, f, mycount) AS`
`       SELECT  n, f, mycount `
`       FROM (SELECT 0,0,0) AS a (n,f,mycount)`
`     UNION ALL`
`       SELECT n, f, mycount `
`       FROM (SELECT 1,1,0) AS b (n, f, mycount)`
`     UNION ALL`
`       SELECT n+1, p.f + pp.f, p.mycount + 1`
`       FROM fibonacci AS p, fibonacci AS pp`
`       WHERE (p.n - 1) = pp.n`
`       AND   p.mycount <= 100;`