How to create fast database queries

Cumulative values

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From Stack Overflow:

There is a table that stores signal statuses that come from different devices.

  • SS1 and SS2 signals are inserted to table in random times
  • If either of SS1 and SS2 signal statuses is up, then the resulting signal should be up
  • If both SS1 and SS2 signal statuses are down, then resulting signal should be down

I want to prepare a query that shows the result signal status changes according to SS1 and SS2 signals

Each record deals with only one signal type here: either SS1 or SS2. To obtain the signal statuses we should query the cumulative values of previous records.

If a record describes a change in SS2, we should query for the most recent change to SS1 that had been recorded so far to obtain the SS1's current status.

In systems other than Oracle, the previous value of a signal could be easily queried using subselects with TOP / LIMIT clauses. But Oracle does not support correlated queries nested more than one level deep, and limiting a subquery result to a single record (which is required by a subquery) requires it (ORDER BY should be nested). This makes constructing such a subquery in Oracle quite a pain.

However, in Oracle, these things can be queries using analytics functions much more efficiently.

Let's create a sample table:

Table creation details

Written by Quassnoi

January 20th, 2010 at 11:00 pm

Posted in Oracle

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