The Informix implementation of SQL is compatible with standard SQL. Informix SQL is also compatible with the IBM version of the language. However, Informix SQL contains extensions to the standard; that is, extra options or features for certain statements, and looser rules for others. Most of the differences occur in the statements that are not in everyday use. For example, few differences occur in the SELECT statement, which accounts for 90 percent of SQL use.

However, the Informix SQL extensions do exist and create conflicts. Thousands of customers have embedded Informix-style SQL in programs and stored routines. Now that Informix products are part of the IBM Data Management product suite, users rely on IBM to keep the Informix-style SQL language the same. Other customers require the ability to use databases in a way that conforms exactly to the ANSI standard, and they rely on IBM to change its language to conform.

IBM resolves the conflict with the following compromise:

• The Informix-style version of SQL, with its extensions to the standard, is available by default.
• You can ask any Informix-style SQL language processor to check your use of SQL and post a warning flag whenever you use an Informix-style extension.

When a difference exists between Informix and ANSI standard, the IBM Informix: Guide to SQL Syntax identifies the Informix syntax as an extension to the ANSI standard for SQL.