The "With" Clause in Oracle SQL | Temporary Recursive Tables | CTE -Common Table Expression - PLSQL
The `WITH` clause, also known as a Common Table Expression (CTE), in Oracle SQL allows you to define temporary result sets within a SELECT, INSERT, UPDATE, or DELETE statement. When it comes to recursion, you can use a CTE to create temporary recursive tables. Here's how you can use the `WITH` clause for recursive tables:
```sql
WITH recursive_cte (column1, column2, ...) AS (
-- Anchor member (initial query)
SELECT column1, column2, ...
FROM your_table
WHERE condition
UNION ALL
-- Recursive member (subsequent queries)
SELECT column1, column2, ...
FROM your_table
JOIN recursive_cte ON your_table.link_column = recursive_cte.link_column
WHERE additional_condition
)
SELECT
FROM recursive_cte;
```
Explanation:
1. **WITH Clause:*
`recursive_cte` is the name given to the Common Table Expression.
`column1, column2, ...` are the columns you want in your temporary result set.
2. *Anchor Member:*
The initial query that defines the starting point of recursion.
In the example, it's the first `SELECT` statement before the `UNION ALL`.
It retrieves the base or anchor rows that meet the specified condition.
3. *UNION ALL:*
Combines the results of the anchor and recursive members.
`UNION ALL` is used because it allows duplicate rows in the result set.
4. *Recursive Member:*
The subsequent query that references the CTE itself.
It defines how to join the CTE with the original table to continue the recursion.
The `JOIN` condition typically involves a column from the table and the CTE.
5. *Final SELECT:*
Retrieves all rows from the recursive CTE.
This structure allows you to perform recursive operations on tables, such as traversing hierarchical data (like organizational charts or bill of materials) or dealing with self-referencing tables.
Note: The `RECURSIVE` keyword is optional in Oracle SQL. You can use it for clarity, but it's not strictly necessary for recursive CTEs.