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Apache MetaModel offers three different ways of defining a query for a DataContext to retrieve data. Each approach offers a set of features, making them appropriate for different situations.

1 - The Query object

At the core of all query execution in MetaModel is the Query object. Query is a regular Java class containing the structural elements that make up a query. The main elements of a query is a set of clauses (SelectClause, FromClause and so on) which in turn each contain a set of items (SelectItem, FromItem etc.).

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• Creating the query this way is a bit verbose, and as we shall see below, it can be made simpler in most cases.

2 - The Query-builder API

To make query definition easier and more developer-guided, Apache MetaModel also has the so-called Query-builder API. This term is used for the set of methods extending from a call to DataContext.query(), ending with either .toQuery() or .execute(). Using the same simple example as before, we can define and execute the query like this:

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• Some features are not available via this API, for instance sub-querying.

3 - Query parsing

Finally, Apache MetaModel allows you to simply parse a query from a string form. You can use this feature in a two-step parse-then-execute mode or in a single step to parse-and-execute.

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