# relational algebra

(redirected from Semijoin)

## relational algebra

(database, theory)
A family of algebra with a well-founded semantics used for modelling the data stored in relational databases, and defining queries on it. The main operations of the relational algebra are the set operations (such as union, intersection, and cartesian product), selection (keeping only some lines of a table) and the projection (keeping only some columns).

The relational data model describes how the data is structured.

Codd's reduction algorithm can convert from relational calculus to relational algebra.

## relational algebra

(1) The branch of mathematics that deals with relations; for example, AND, OR, NOT, IS and CONTAINS.

(2) In a relational database, a collection of rules for dealing with tables; for example, JOIN, UNION and INTERSECT.
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References in periodicals archive ?
[15] Martinez-Bazan, Norbert and Dominguez-Sal, David, "Using semijoin programs to solve traversal queries in graph databases," in Proc.
Optimization techniques, such as lightweight and fine-grained sideways information passing, semijoin, and other join processing optimizations, were incorporated to further enhance the performance of the query processing engine.
For a proxy relation that has no conditions, the retrieval conditions are created using data from a local relation or a proxy relation with conditions based on a join condition that join the two relations (the semijoin operation).
2.3.5 Semijoins. Semijoin programs were proposed as another technique to process joins between tables stored at different sites [Bernstein et al.
Additional Algebraic Operators Operator Description [??.sub.p] semijoin with predicate p [[bar]??.sub.p] antisemijoin with predicate p MATHEMATICAL EXPRESSION attribute rename NOT REPRODUCIBLE IN ASCII] E[X = expr] attribute extension and expression evaluation A[X = a(A);B] attribute extension and aggregate function evaluation 2.1.1 Antisemijoin.
These techniques include (a) naive tuple substitution, (b) relational text processing, (c) semijoin, and (d) probing.
For the special case where the traced tuple set is the entire view table V (which will appear later in our recursive tracing algorithm for general views), we use a flag "ALL" to specify that the entire view table is to be traced, and the tracing query can be simplified by removing the semijoin
From this figure, we can see that the main features our model lacks are tuples, semijoin by content (e.g., to retrieve all chapters whose titles appear in this paragraph), and the possibility of having overlaps and combining nodes in the result set of a query.
Optimal semijoin schedules for query processing in local distributed database systems.
Another unexplored and, we believe, promising research direction is the application of our framework of operator tree / query graph / association tree to reorder other operators that combine relations, such as semijoin and antijoin.
Optimal semijoin schedules for query processing in local distributed database systems.In Proceedings of ACM SIGMOD International Conference on the Management of Data (Ann Arbor, Mich., Apr.

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