Editing Data access object (section)

{{Short description|Abstract interface to a computer database}}
{{About|the software design pattern|the Microsoft library|Jet Data Access Objects}}

In [[software]], a '''data access object''' ('''DAO''') is a pattern that provides an abstract [[Interface (computer science)|interface]] to some type of [[database]] or other [[Persistence (computer science)|persistence mechanism]]. By mapping application calls to the persistence layer, the DAO provides data operations without exposing database details. This isolation supports the [[single responsibility principle]]. It separates the data access the application needs, in terms of domain-specific objects and data types (the DAO's public interface), from how these needs can be satisfied with a specific [[Database Management System|DBMS]] (the implementation of the DAO).

Although this [[design pattern]] is applicable to most programming languages, most software with persistence needs, and most databases, it is traditionally associated with [[Java Platform, Enterprise Edition|Java EE]] applications and with [[relational databases]] (accessed via the [[JDBC]] API because of its origin in [[Sun Microsystems]]' best practice guidelines<ref>
{{cite web
 | title = Core J2EE Patterns - Data Access Objects
 | publisher = Sun Microsystems Inc.
 | url = http://www.oracle.com/technetwork/java/dataaccessobject-138824.html
 | date = 2007-08-02
}}</ref> "Core J2EE Patterns".

This object can be found in the [[Data_access_layer|Data Access layer]] of the [[Multitier_architecture|3-Tier Architecture]].

There are various ways in which this object can be implemented:
* One DAO for each table.
* One DAO for all the tables for a particular DBMS.
* Where the SELECT query is limited only to its target table and cannot incorporate JOINS, UNIONS, subqueries and Common Table Expressions (CTEs)
* Where the SELECT query can contain anything that the DBMS allows.