Columns, column sets and keys

Explains the concepts of columns, column sets and keys.

A database contains a collection of tables. A table contains a set of rows. A row contains a set of columns. Every row in a table conforms to a structure described by the table's column set. Each column has attributes such as a name, a type, a maximum length, etc. Column types include every type of integer, real, date-time and text supported by the Symbian platform. In addition, a long binary column (sometimes known as BLOBs) may be written using the stream interface, allowing stream-aware components to use a database column for persistence.

Auto-increment columns support automatic key or id generation. Column names within a single table must be unique, and follow the rules for DBMS names. A column definition is encapsulated by the TDbCol class, and the set of columns which describes the table in the CDbColSet class.

Every index in a database is defined by its key, represented by the CDbKey class. Tables can index certain columns to sort their rows into a particular order, or provide fast key-based access to their rows. A key has attributes such as being unique or primary, a comparison specification for text columns, and a list of columns which make up the key, the TDbKeyCol class.

See also:

• Rules for DBMS names