Please explain ranking and score?

I am familiar with basic SQL and PL/SQL.  I met the following clause in a query, I do not understand:

SELECT...

T1.valueA,

T1. DateValue,

T1.valueB,

T1.dateValue_2,

IsCurrent

(SELECT
Decode (ROW_NUMBER () over)
(order by T1.dateValue ASC T1.valueA partition), 1, DECODE (sign (T1.dateValue - sysdate), 1, ' n 'Y'), 'n') as IsCurrent
"N" as IsCancellable
dense_rank () over (partition T1.valueA order by T1.valueB desc, T1.dateValue desc, T1.dateValue_2 desc) as rank

T1 etc...

If it's just a way of ordering within groups.

No - no groups. The partition clause is about the creation of 'sets', not groups. The word 'group' has a very specific meaning in SQL and it is important that it be not used incorrectly or people are going to get confused.

When the data are combined the result often put less than rows as there are in the source data because data are aggregated.

When the data is partitioned any line is destroyed or created but data "sets" are produced.

Tags: Database

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