Key Concepts

The Doctrine Query Language (DQL) is an object query language. It let's you express queries for single objects or full object graphs, using the terminology of your domain model: class names, field names, relations between classes, etc. This is a powerful tool for retrieving or even manipulating objects without breaking the separation of the domain model (field names, class names, etc) from the relational model (table names, column names, etc). DQL looks very much like SQL and this is intended because it makes it relatively easy to grasp for people knowing SQL. There are, however, a few very important differences you should always keep in mind:

Take this example DQL query:

FROM User u 
LEFT JOIN u.Phonenumbers where u.level > 1

The things to notice about this query:

• We select from classes and not tables. We are selecting from the User class/model.
• We join along associations (u.Phonenumbers)
• We can reference fields (u.level)
• There is no join condition (ON x.y = y.x). The associations between your classes and how these are expressed in the database are known to Doctrine (You need to make this mapping known to Doctrine, of course. How to do that is explained later in the Defining Models chapter.).

DQL expresses a query in the terms of your domain model (your classes, the attributes they have, the relations they have to other classes, etc.).

It's very important that we speak about classes, fields and associations between classes here. User is not a table / table name . It may be that the name of the database table that the User class is mapped to is indeed named User but you should nevertheless adhere to this differentiation of terminology. This may sound nit picky since, due to the ActiveRecord approach, your relational model is often very similar to your domain model but it's really important. The column names are rarely the same as the field names and as soon as inheritance is involved, the relational model starts to diverge from the domain model. You can have a class User that is in fact mapped to several tables in the database. At this point it should be clear that talking about "selecting from the User table" is simply wrong then. And as Doctrine development continues there will be more features available that allow the two models to diverge even more.