Relational division and natural composition are similarly related to the boolean-valued set operations of superset and overlap, respectively, and are incorporated by Aldat into a family of twelve sigma-joins, along with extensions of other set comparisons (proper superset, set equality, subset and proper subset, and the six complementary comparisons).
The unary operators of the original relational algebra, projection and restriction, are extended to projection and selection, and unified into the T-selector, which selects any tuple whose attributes satisfy a given, arbitrary, boolean expression, and then projects on another specified set of attributes. The T-selector is further generalized to the QT-selector, which adds quantifiers based on counting and proportion-of operators. QT-selectors appear to have straightforward translations into natural language queries, except when they become too precise for ready expression in everyday speech.
A second family of unary operators are the editors, some general, others specialized to treat particular relations as constructs such as spreadsheets, pictures, or prolog fact bases. These are Janus-like operators which from the viewpoint of the Aldat programmer are unary operators such as projection, but from the perspective of the end-user are interactive programs with command languages, graphical user interfaces, and any other handy facility for working with the individual components of the data.
The relational algebra provides an abstraction over looping, and has no notion of tuples to tempt the programmer to lower levels of thought. (There is a non-deterministic pick operator which guarantees a singleton (or empty) relation as its result, for the programmer who must escape from the higher-level constructs.)
The vertical operations allow aggregations, using commutative and associative operators. Counting and summing are simple consequences, and from these, means, standard deviations, etc. So are maximum- and minimum-finding, and aggregative and (for all), or (for some), product, etc. These are called reductions, after the APL meta-operator of the same name and similar function. There are also classes of equivalence reduction, which apply reduction to subrelations that are grouped by the values of some (set of) attributes, of functional mapping, which induce an order on a relation according to the values of a (set of) attributes and permit aggregative operators which are not associative or commutative, and of partial functional mapping, which support both order and grouping.
All domain algebra operations are independent of any particular relation, and so produce virtual attributes. These may be actualized for specific relations through the relational algebra. The domain algebra also abstracts over looping.
Aldat has a deferred assignment which produces a view: a relation of no extent which is evaluated by executing the view each time the left-hand operand is required. A view can be seen as a parameterless function of its right-hand operands.
Views may be recursive, with the left operand defined in terms of itself on the right, or mutually recursive, with the left operand defined in terms of other views, which in turn are ultimately defined in terms of that same left operand. Such recursion can be used to specify transitive closures and rule-based inference engines.
The recursion is implemented in the most naive way because this makes it general and because mutual recursion, even naively implemented, can express the most sophisticated implementations of simple recursion such as transitive closure.
The domain algebra, working independently of the relational algebra, can be combined with recursion to support numerical and other calculations on the attributes of the recursively defined relations. An example is the bill-of-materials problem, or a sophisticated inference engine which allows arithmetic operations. General flow problems may be expressed, on cyclic or acyclic graphs.
The invocation mechanism is a T-selector on the computation, or a join of the computation with an (extrinsic) relation. The code in the computation is executed for each tuple of the relation. Joins of computations with each other can also be interpreted usefully.
The motivation for using computations, rather than functions or procedures, in Aldat is to make available the same syntax for using intrinsic relations (computations) as extrinsic relations (relations). A practical result is that a model for making predictions has the same syntactic form as a database storing the historical data: the same query can be used for the future as for the past.
Computations need not be functional, but may have local states, so that subsequent invocations depend on a memory of preceeding ones. They may also be called recursively.
In particular, this supports data abstraction and allows relational attributes to be defined on complex types.
States are the fundamental aspect of ``object orientation''. The need to create instances, automatically, of any class with state follows from this, and is the primary accomplishment of object orientation. In a database, everything is state because databases are persistent. Aldat computations support state in the object-oriented sense of hidden state. The ordinary relations of the database provide the setting for instantiation of these states, and Aldat allows the programmer to instantiate by joining the computation-with-state to an appropriate relation. This removes the need for a new operator, which would be cumbersome in the database context of possibly millions of instantiations.
From instantiation follow classes, and from these, subclasses and inheritance. Aldat sees inheritance as syntactic sugar for a join between the inheriting relation and the bequeathing relation. Of course, once we have the syntactic sugar, we can also have a special implementation (which the programmer does not see) replacing the join implementation by pointer dereferencing and capturing the speed advantage of object-oriented databases in the very special cases to which they apply.
Computations may inherit from each other in the same way, by joins defined on computations, giving the benefits of method inheritance with the same syntax and mechanism as data inheritance.
(There is thus nothing new in either nesting or inheritance: everything they can do can be done in principle by the relational and domain algebras of Aldat. But the special implementations underlying the specialized syntax of object orientation can make these special operation much faster than the general algebras.)
This is the basis for active databases, which respond actively to events such as updates, rather than just passively accepting them.
Non-determinism is kept independent of synchronization in this approach (unlike Linda), and is implemented by the pick operator.
Technical overview and applications ( PostScript 243K)
Relational database programming in Aldat (PostScript 147K)
Attribute Metadata
for Relational OLAP and Data Mining (PostScript 235K)
(Metadata of type
Eighth Biennial Workshop on Data Bases and Programming Languages (DBPL'01),
Monteporzio Catone - Roma, Italy, Sept. 8--10, 2001.