How to create fast database queries

Archive for June 20th, 2009


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From Stack Overflow:

Are there non obvious differences between NVL and COALESCE in Oracle?

The obvious differences are that COALESCE will return the first non-NULL item in its parameter list whereas NVL only takes two parameters and returns the first if it is not NULL, otherwise it returns the second.

It seems that NVL may just be a base case version of COALESCE.

Am I missing something?

In case of two agruments of same type, these functions are basically the same: they both return first non-NULL values.

However, they are implemented differently:

  • NVL always evaluates both arguments
  • COALESCE doesn't evaluate anything after it finds the first non-NULL argument

This behavior can be utilized.

Here's a real world example from a restaurant management system I developed about 10 years ago.

A bill that is presented to a customer consists of the items he or she ordered.

Majority of the dishes ordered are plain menu items: something that has its price printed in the menu.

However, there are some special things which are not included to the menu: a customer may want to order something exclusive and pay for it.

The manager should estimate the cost of ingredients, time and effort to make this dish, then calculate the price and name it: everything in 30 seconds.

These things are of course come overpriced, but the customers that make such orders don't seem to care much about money.

In the bill that appears as special order. When a manager enters it into the computer, she should also enter the price she had calculated.

These prices then get back to the server and are stored in a table (not the one where regular menu prices are stored).

At the end of month, a report to the month's receipts should be generated.

Here are the tables containing the data relevant to the reports. They of course are simplified, but the principle remains the same.
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Written by Quassnoi

June 20th, 2009 at 11:00 pm

Posted in Oracle