Description
Like Oracle's PL/SQL or Microsoft's Transact-SQL, DAL is essentially an extended version of SQL supporting basic query functionality and adding clean syntax for cursor operations, logic, and loops.
When sent a command, early versions of Apple's DAL interpreter broke down the statement and re-built it into subqueries for the underlying data sources. This translation took place on the server-side, just like PL/SQL and Transact-SQL, but required a fairly expensive "adaptor" program of often dubious performance. This adaptor made DAL considerably less appealing than later systems like ODBC, where the translation normally takes place on the client side and is typically included for free with the database engine. The downside to the ODBC approach is that, theoretically at least, more network bandwidth is used up to pull the "raw data" to the client machine for processing back into a standard format.
On the client end, DAL was originally accessed directly through a system extension, but DAL was later rolled into a single ODBC-like driver layer, the Data Access Manager (DAM). DAM was ODBC-like in concept, but did not include the SQL layers, it was strictly a system for sending "opaque" queries and receiving result sets. DAM also included the concept of a "query document" that allowed the DAL (or other) queries to be written in an authoring system and then easily used in any client application.
Read more about this topic: Data Access Language
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