Editing SQL (section)

===Reasons for incompatibility===
Several reasons for the lack of portability between database systems include:

* The complexity and size of the SQL standard means that most implementers do not support the entire standard.
* The SQL standard does not specify the database behavior in some important areas (e.g., [[index (database)|indices]], file storage), leaving implementations to decide how to behave.
* The SQL standard defers some decisions to individual implementations, such as how to name a results column that was not named explicitly.<ref name="professionals">{{cite book |last1=Date |first1=Chris J. |title=Relational Theory for Computer Professionals: What Relational Databases are Really All About |date=2013 |publisher=O'Reilly Media |location=Sebastopol, Calif |isbn=978-1-449-36943-9 |edition=1.}}</ref>{{rp|207}}
* The SQL standard precisely specifies the syntax that a conforming database system must implement. However, the standard's specification of the semantics of language constructs is less well-defined, leading to ambiguity.
* Many database vendors have large existing customer bases; where the newer version of the SQL standard conflicts with the prior behavior of the vendor's database, the vendor may be unwilling to break [[backward compatibility]].
* Little commercial incentive exists for vendors to make changing database suppliers easier (see [[vendor lock-in]]).
* Users evaluating database software tend to place other factors such as performance higher in their priorities than standards conformance.