Tables
The table is the basic data structure used in a relational database. A table is a collection
of rows. Each row in a table contains one or more columns. If you??™re unfamiliar
with relational databases, youcan map a table to the concept of a file or database in
a nonrelational database, just as you can map a row to the concept of a record in a
nonrelational database.
As of Oracle9i, youcan define external tables. As the name implies, the data for an
external table is stored outside the database, typically in a flat file. The external table
Should You Use NULLs?
The idea of three-state logic may seem somewhat confusing, especially when you imagine
your poor end users executing ad hoc queries and trying to account for a value
that??™s neither TRUE nor FALSE. This prospect may concern you, so you may decide
not to use NULL values at all.
We believe that NULLs have an appropriate use. The NULL value covers a very specific
situation: a time when a column has not had a value assigned. The alternative to
using a NULL is using a value with another meaning??”such as 0 for numbers??”and
then trying to somehow determine whether that value has actually been assigned or
simply exists as a replacement for NULL.
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