SQL WITH clause tricks

http://www.dba-oracle.com/t_sql99_with_clause.htm

Starting in Oracle9i release 2 we saw an incorporation of the SQL-99 WITH clause (a.k.a. subquery factoring), a tool for materializing subqueries to save Oracle from having to re-compute them multiple times.

The SQL WITH clause is very similar to the use of Global temporary tables (GTT), a technique that is often used to improve query speed for complex subqueries. Here are some important notes about the Oracle WITH clause:

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