SQL subquries

Baan SQL permits the use of subqueries. These are SELECT statements in the WHERE clause of another SELECT statement.Defining nested queries can be very difficult. It is best to define the subquery of the lowest level first and the main question last. 
Example 1
Select those prices from the item file that are above average. When calculating the average, the system should not take prices less than or equal to zero into account.
SELECT tiitm001.copr | cost price
FROM tiitm001
WHERE tiitm001.copr >
( SELECT avg(tiitm001.copr)
WHERE tiitm001.copr > 0 )
ORDER BY tiitm001.copr
In this case, the subquery should only produce one result (here the average cost price). If the subquery produces more than one result, use the operators IN and EXISTS.

Example 2
Select the numbers and names of all suppliers who have yet to deliver.
SELECT tccom020.suno, tccom020.nama
FROM tccom020 | Suppliers
FROM   timps053 | Purchase orders
WHERE  timps053.suno = tccom020.suno )

SELECT tccom020.suno, tccom020.nama
FROM tccom020
WHERE tccom020.suno IN
( SELECT   timps053.suno
FROM     timps053
GROUP BY timps053.suno )

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