Plain SQL vs Dialects

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.

Edit Jan 13

Thanks for all of your answers : they have conveyed to me an impression that a lot of the DBMS-specific SQL is created to allow work-arounds for poor relational design. Which leads me to the conclusion you probably wouldn't want to port most existing applications.

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.

Answer 2

Oracle has some additions, such as model or hierarchical queries that are very difficult, if not impossible, to translate into pure SQL

Answer 3

Even when SQL:2008 can do something sometimes the syntax is not the same. Take the REGEXP matching syntax for example, SQL:2008 uses LIKE_REGEX vs MySQL's REGEXP.

And yes, I agree, it's very annoying.

Answer 4

Part of the problem with Oracle is that it's still based on the SQL 1992 ANSI standard. SQL Server is on SQL 1999 standard, so some of the things that look like "extensions" are in fact newer standards. (I believe that the "OVER" clause is one of these.)

Oracle is also far more restrictive about placing subqueries in SQL. SQL Server is far more flexible and permissive about allowing subqueries almost anywhere.

SQL Server has a rational way to select the "top" row of a result: "SELECT TOP 1 FROM CUSTOMERS ORDER BY SALES_TOTAL". In Oracle, this becomes "SELECT * FROM (SELECT CUSTOMERS ORDER BY SALES_TOTAL) WHERE ROW_NUMBER <= 1".

And of course there's always Oracle's infamous SELECT (expression) FROM DUAL.

Edit to add:

Now that I'm at work and can access some of my examples, here's a good one. This is generated by LINQ-to-SQL, but it's a clean query to select rows 41 through 50 from a table, after sorting. It uses the "OVER" clause:

SELECT [t1].[CustomerID], [t1].[CompanyName], [t1].[ContactName], [t1].[ContactTitle], [t1].[Address], [t1].[City], [t1].[Region], [t1].[PostalCode], [t1].[Country], [t1].[Phone], [t1].[Fax]
    FROM (
        SELECT ROW_NUMBER() OVER (ORDER BY [t0].[ContactName]) AS [ROW_NUMBER], [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Phone], [t0].[Fax]
        FROM [dbo].[Customers] AS [t0]
        ) AS [t1]
    WHERE [t1].[ROW_NUMBER] BETWEEN 40 + 1 AND 40 + 10
    ORDER BY [t1].[ROW_NUMBER]

Answer 5

Common here on SO

• ISNULL (SQL Server)
• NVL ('Orable)
• IFNULL (MySQL, DB2?)
• COALESCE (ANSI)

To answer exactly:

ISNULL can easily give different results as COALESCE on SQL Server because of data type precedence, as per my answer/comments here