Defines column, column set, and index key structures.
The API has three key concepts: column, column set, and index key.
A table in a database is defined by a set of columns. Each column
has attributes such as a name, a type, and a maximum length (if text or
binary).
A column definition is encapsulated by
TDbCol.
A set of columns that describe a table are encapsulated in
CDbColSet. The set can be iterated over using
TDbColSetIter.
An index key orders one or more table columns (keys). It 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 index key is encapsulated in CDbKey. A
column for the key is encapsulated in TDbKeyCol.