Programming the Perl DBI

Query Languages and Data Functions

Database operations can be split into those manipulating the database itself (that is, the logical and physical structure of the files comprising the database) and those manipulating the data stored within these files. The former topic is generally database-specific and can be implemented in various ways, but the latter is typically carried out by using a query language.^[4]

All query languages, from the lowest level of using Perl’s string and numerical handling functions to a high-level query language such as SQL, implement four main operations with which you can manipulate the data. These operations are:

Fetching

The most commonly used database operation is that of retrieving data stored within a database. This operation is known as fetching, and returns the appropriate data in a form understood by the API host language being used to query the database. For example, if you were to use Perl to query an Oracle database for data, the data would be requested by using the SQL query language, and the rows returned would be in the form of Perl strings and numerics. This operation is also known as selecting data, from the SQL SELECT keyword used to fetch data from a database.

Storing

The corollary operation to fetching data is storing data for later retrieval. The storage manager layers translate values from the programming language into values understood by the database. The storage managers then store that value within the data files. This operation ...