All programming languages have standards which web developers should be aware, and SQL isn’t any different. SQL was standardized by ANSI and then ISO, with new revisions to the language being occasionally submitted. The latest revision is SQL:2008, although the most important revision that developers should be aware of is SQL:1999. The 1999 revision introduced recursive queries, triggers, support for PL/SQL and T-SQL, and a few newer features. It also defined that the JOIN statements be done in the FROM clause, as opposed to the WHERE clause.
When writing code, it is important to keep in mind why standards-compliant code is useful. There are two primary reasons why standards are used. The first is maintainability, and the second is cross-platform standardization. As with desktop applications, it is assumed that websites will have long lifespans, and will go through various updates to add new functionality and repair problems. As any systems analyst will tell you, systems spend a majority of their lifespan in the maintenance phase. When a different programmer accesses your code in 2, 5 or 10 years, will they still be able to understand what your code is doing? Standards and comments are designed to promote maintainability.
The other reason is cross-platform functionality. With CSS, there is currently an ongoing standards battle between Firefox, Internet Explorer, Chrome, and other browsers about the interpretation of code. The reason for the SQL standards is to prevent a similar situation between Oracle, Microsoft and other SQL variants such as MySQL.