Basics
Queries are used for fetching a list of objects from the database. Queries will use data base mechanism for perfomant data access. Queries will deliver a list of objects of a certain Metaclass. Queries can be used in different scenarios:
- data fetching (n records)
- finding a single or a set of recordset(s)
- used as definition of root nodes in a View
Query, QueryItem
A Query is defined for a single MetaClass. The MetaClass is defined by a Referenceproperty (“FILTEROBJECT”). A Query will deliver objects (instances) of the defined MetaClass. A Query holds a list of QueryItems. A single QueryItem defines a single filter rule for the evaluation of the result collection:
- OPERATOR
e.g. “=”
- LIKECOMPARISON
is the given item a “LIKE” filter criteria (wildcard is added to the filtervalue)
- FILTERVALUE
the value the objects are compared to
- ISNULL
searched for NULL values
- LOGICALAND
is there a logical AND relation between the given QueryItem and its predecessor (TRUE) or an OR relation (false)
Relation between Query and Items
The correspondence between Queries and its items is created and stored by a relationship (“QUERYRELATION”). Therefore a single QueryItem can be used several times for different Queries.
Usage of DatabaseViews
It is also possible to define a Database View and use this View for the evaluation of Queries (see VirtualContentClass).