David,
Tuesday, April 16, 2002, 3:01:46 PM, you wrote:

DA> Cards on the table,  I have an assignment in soon and part of the
DA> deliverable is to design some relational algebra tree's based on some
DA> queries I have been given.

DA> This is one of the queries:
DA> SELECT id
DA> FROM Table1
DA> WHERE id NOT IN
DA>    (SELECT id
DA>    FROM Table2);

DA> I have an idea that this query uses a left join after projecting id in
DA> both tables.

DA> If anyone can help me understand this a bit better, I would REALLY
DA> appreciate an email from you.

See:

SELECT table1.id FROM table1 LEFT JOIN table2 ON table1.id=table2.id
WHERE table2.id IS NULL;


DA> Thanks lots
DA> David Ayliffe





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