# SDO Relational Data Access Service

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In order to use the Relational Data Access Service for Service Data Objects, you will need to understand some of the concepts behind SDO: the data graph, the data object, the disconnected way of working, the change summary, XPath and property expressions, and so on. If you are not familiar with these ideas, you might want to look first at the section on SDO. In addition, the Relational DAS makes use of the PDO extension to isolate itself from specifics of different back-end relational databases. In order to use the Relational DAS you will need to be able to create and pass a PDO database connection; for this reason you might also want to take a look at the section on PDO.

The job of the Relational DAS is to move data between the application and a relational database. In order to do this it needs to be told the mapping between the database entities - tables, columns, primary keys and foreign keys - and the elements of the SDO model - types, properties, containment relationships and so on. You specify this information as metadata when you construct the Relational DAS.

## Overview of Operation

1. The first step is to call the Relational DAS's constructor, passing the metadata that defines the mapping between database and SDO model. There are examples of this below.

2. The next step might be to call the `executeQuery` or `executePreparedQuery` methods on the Relational DAS, passing either a literal SQL statement for the DAS to prepare and execute, or a prepared statement with placeholders and a list of values to be inserted. You may also need to specify a small amount of metadata about the query itself, so that the Relational DAS knows exactly what columns will be returned from the database and in what order. You will also need to pass a PDO database connection.

   The return value from `executeQuery` or `executePreparedQuery` is a normalised data graph containing all the data from the result set. For a query that returns data obtained from a number of tables, this graph will contain a number of data objects, linked by SDO containment relationships. There may also be SDO non-containment references within the data.

   Once the query has been executed and the data graph constructed, there is no need for either that instance of the Relational DAS or the database connection. There are no locks held on the database. Both the Relational DAS and the PDO database connection can be garbage collected.

3. Quite possibly the data in the data graph will go through a number of modifications. The data graph can be serialised into the PHP session and so may have a lifetime beyond just one client-server interaction. Data objects can be created and added to the graph, the data objects already in the graph can be deleted, and data objects in the graph can be modified.

4. Finally, the changes made to the data graph can be applied back to the database using the `applyChanges` method of the Relational DAS. For this, another instance of the Relational DAS must be constructed, using the same metadata, and another connection to the database obtained. The connection and the data graph are passed to `applyChanges`. At this point the Relational DAS examines the change summary and generates the necessary INSERT, UPDATE and DELETE SQL statements to apply the changes. Any UPDATE and DELETE statements are qualified with the original values of the data so that should the data have changed in the database in the meantime this will be detected. Assuming no such collisions have occurred the changes will be committed to the database. The application can then continue to work with the data graph, make more changes and apply them, or can discard it.

There are other ways of working with the data in the database: it is possible to just create data objects and write them to the database without a preliminary call to `executeQuery`, for example. This scenario and others are explored in the Examples section below.

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