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The result of an SQL query is a set of rows with a fixed number of columns. The result of an EJB QL query is either a single object, a collection of entity objects of a given type, or a collection of values retrieved from CMP fields. One has to understand the data model of enterprise beans in order to write effective queries. [3]
Entity Beans before EJB 2.0 should not be used in great numbers [3] because each entity bean was in fact a RMI stub with its own RMI connection to the EJB server. Obtaining 1000 entity beans as a single operation would result in 1000 simultaneous internet connections to the RMI back-end [ citation needed ] .
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Jakarta Enterprise Beans (EJB) specification defines a set of lightweight APIs that an object container (the EJB container) will support in order to provide transactions (using JTA), remote procedure calls (using RMI or RMI-IIOP), concurrency control, dependency injection and access control for business objects. This package contains the ...
EJB timer service (new): Event-based mechanism for invoking EJBs at specific times. Message-driven beans accepts messages from sources other than JMS. Message destinations (the same idea as EJB references, resource references, etc.) has been added. EJB query language (EJB-QL) additions: ORDER BY, AVG, MIN, MAX, SUM, COUNT, and MOD.
Title Authors ----- ----- SQL Examples and Guide 4 The Joy of SQL 1 An Introduction to SQL 2 Pitfalls of SQL 1 Under the precondition that isbn is the only common column name of the two tables and that a column named title only exists in the Book table, one could re-write the query above in the following form:
A query language, also known as data query language or database query language (DQL), is a computer language used to make queries in databases and information systems. In database systems, query languages rely on strict theory to retrieve information. [1] A well known example is the Structured Query Language (SQL).
A hierarchical query is a type of SQL query that handles hierarchical model data. They are special cases of more general recursive fixpoint queries, which compute transitive closures . In standard SQL:1999 hierarchical queries are implemented by way of recursive common table expressions (CTEs).