Query of Query(cursor)

Question

Hi all,
I want to implement something like "query of queries" in Oracle with the help of CURSORS.
Is it possible to use a cursor in the from clause of a query?
Please help me....
Thanking u all in advance...
Regards,
Aswathy.

Answer 1

> But can i know if this can be done with CURSORS.
A cursor is not a data set or result set - it is not a copy of rows (or row identifiers) in memory.
A cursor is (SQL) program parsed & compiled into an execution plan. This contains the instructions to fetch the applicable rows.
A FETCH from a cursor is an instruction to Oracle to execute that SQL program to return the next rows (or rows in the case of a bulk fetch).
It seems to me that you are thinking of row-by-row processing - which is why you want a query of a query and use a cursor for that.
Row-by-row processing is also aptly named slow-by-slow processing.
The correct approach is to think data sets. And write queries to deal with data sets. And yes, you can create a data set using SQL and re-use it with the WITH syntax.
For example (and a very poor example as analytical SQL should be used instead, but the intention is simply to show you how the WITH syntax works):
with MY_DATA_SET as(
select
col1,
col2,
col3
from table1
where <some conditions & filters>
select
'MAX C1' as NAME,
max(col1)
from my_data_set
union all
select
'MAX C2' as NAME,
max(col2)
from my_data_set
union all
select
'C1=C2' as NAME,
count(*)
from my_data_set
where c1 = c2
<etc>
You can create multiple "data sets" like this and join them, further refine them, etc. Just remember to aim to write minimal SQL code... the less code there is, the less complexity, the less work to do (usually) and the better the likelihood of good performance and scalability.