Question

DBMS Vendors use SQL dialect features to differentiate their product, at the same time claiming to support SQL standards. ‘Nuff said on this.

Is there any example of SQL you have coded that can’t be translated to SQL:2008 standard SQL ?

To be specific, I’m talking about DML (a query statement), NOT DDL, stored procedure syntax or anything that is not a pure SQL statement.

I’m also talking about queries you would use in Production, not for ad-hoc stuff.

Answer 1

Typical differences include subtly differnt semantics (for example Oracle handles NULLs differently from other SQL dialects in some cases), different exception handling mechanisms, different types and proprietary methods for doing things like string operations, date operations or hierarchical queries. Query hints also tend to have syntax that varies across platforms, and different optimisers may get confused on different types of constructs.

One can use ANSI SQL for the most part across database systems and expect to get reasonable results on a database with no significant tuning issues like missing indexes. However, on any non-trivial application there is likely to be some requirement for code that cannot easily be done portably.

Typically, this requirement will be fairly localised within an application code base – a handful of queries where this causes an issue. Reporting is much more likely to throw up this type of issue and doing generic reporting queries that will work across database managers is very unlikely to work well. Some applications are more likely to cause grief than others.

Therefore, it is unlikely that relying on ‘portable’ SQL constructs for an application will work in the general case. A better strategy is to use generic statements where they will work and break out to a database specific layer where this does not work.

A generic query mechanism could be to use ANSI SQL where possible; another possible approach would be to use an O/R mapper, which can take drivers for various database platforms. This type of mechanism should suffice for the majority of database operations but will require you to do some platform-specifc work where it runs out of steam.

You may be able to use stored procedures as an abstraction layer for more complex operations and code a set of platform specific sprocs for each target platform. The sprocs could be accessed through something like ADO.net.

In practice, subtle differences in paramter passing and exception handling may cause problems with this approach. A better approach is to produce a module that wraps the
platform-specific database operations with a common interface. Different ‘driver’ modules can be swapped in and out depending on what DBMS platform you are using.