For example, the following nested SQL query cannot be unnested:
SELECT AVG(salary)
FROM
( SELECT DISTINCT department, salary
FROM emp
)
Christian Kaufmann
Whenever you need aggregated values from a subset of one table when the subset is not build because of the value of one or several attributes (in other words when you didn't use "where" in the select) or of course when the resultset is build of a query over several tables
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Mohsen Biglari
I think:
1. When you need multi-level filtering of a table
2. When you need a value from several tables
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Sonal Chintankumar Bhavsar
you can use Group By Clause
For example above same query you can execute like:
SELECT AVG(salary),department
FROM emp
GROUP BY department.
Try It.
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