Web Developer’s Date/Time Cheatsheet

2 06 2009

If you work in more than one database or  programming language, it is almost impossible to keep all of the different date/time formats straight. As a developer, I found myself going back to the manuals for date/time formats more than anything else.

Here’s a web developer’s date/time cheatsheet. It covers the Oracle, Postgres, MySQL and MS SQL databases; and Java, PHP, Python, Ruby and Flex/ActionScript3 programming languages.

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Timespan – Creating custom data types in Oracle

20 05 2009

This is the Oracle counterpart to my original post on creating a timespan data type in Postgres. I’m not going to rehash what a timespan is or why and where you would use it. But quickly, we will be adding a custom data type to model a span of time. Your work shift starts at x and ends at y and lasted z.

Oracle is a little more object-relational than Postgres when it comes to data types. Here, in typical OOP fashion, we can define a data type that has attributes, member functions, static functions and custom constructors.

Creating a custom type is in many ways like creating a package with a spec and body.
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Timestamp Math in Postgres

20 05 2009

Postgres allows us to add or subtract integers to a DATE type. So ‘2009-01-01’ + 1 = ‘2009-01-02’. But what we can’t do is add NUMERIC to DATE or add anything to TIMESTAMP. So ‘2009-01-01’ + 1.25 and ‘2009-01-01 12:30:00’ + 1 both fail.

I use this feature a lot on Oracle. So lets add that functionality to Postgres.
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Custom sorts

20 05 2009

Problem: You need to do a non natural sort on Postgres. For example, you may want your automotive results sorted Car, Truck, SUV then Motorcycle.

Solution: If we were using MySQL, (and we’re not) we’d do something like ORDER BY find_in_set(category, ‘Car, Truck, SUV, Motorcycle’). That lets you sort on the index position of category in the set. In Postgres, we’ll use arrays instead of sets.
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ETL With Postgres Arrays

20 05 2009

One area where Postgres stands head and shoulders above Oracle is arrays. Oracle arrays/collections are quite cumbersome to use. But Postgres provides us with a simple to use and incredibly power tool in arrays.

Recently I had to do some ETL work were the source was a real mess. (There’s a real surprise.) Its always fun when the database designer graduated from the MS Excel school of normalization.

In this particular table, each row contains up to 8 test scores for 3 terms. So 1 row from the import table will need to be transformed into up to 24 rows which need mapped to the appropriate test and term, while eliminating null entries.

The solution is to use multi-dimentional arrays to map each entry. One dimension will map the term and the second dimension will map the test.
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Timespan – Creating custom data types in PostgreSQL

19 05 2009

Working with time has always been the bane of database development. And one that it seems every DMBS has implemented differently. Postges and MySQL have a DATE type that really just stores the date. Oracle also has a DATE type but it stores time to the second. The equivalent in MySQL is called DATETIME. Postgres doesn’t have a DATETIME, instead you have to use TIMESTAMP(0) (technically TIMESTAMP WITH OUT TIME ZONE, but I like the shorthand personally) And while Oracle and Postgres can handle any reasonable timestamp, MySQL’s TIMESTAMP is limited to the range of the Unix timestamp. Of course MS SQL Server deserves honorable mention as having the absolute worst handling of dates and times.

So after writing dozens of applications that deal with time, I (finally) had an epiphany. Most often I am modeling a span of time with start time and end time or start time and duration. And while a timestamp is ideal for storing a specific point in time (creation time, last login); it is less ideal for recording spans (employee shift, appointment, trip time). And depending on the application (and even the table) some times I would model it as a start time and end time, some times as a start time and duration in minutes or seconds, some times as a start time and interval.

But since Postgres and Oracle make it easy to create custom data types, lets simplify things by bundling the start and end times into a single timespan type. For now we’ll implement it in Postgres and later I’ll show it in Oracle.

There are several ways we could do this in Postgres, but lets stay away from creating types in C. Actually, we don’t even need to use pl/pgsql, as using straight sql seems to perform faster and has the power to do what we need. Read the rest of this entry »