Databases Reference
In-Depth Information
CHAPTER 4
Oracle Data Structures
In the previous chapters, we examined some distinctions between the different com‐
ponents that make up an Oracle Database. For example, we pointed out that the Oracle
instance differs from the files that make up the physical storage of the data in tablespaces,
that you cannot access the data in a tablespace except through an Oracle instance, and
that the instance itself isn't very valuable without the data stored in those files.
The instance is the logical entity used by applications and users, separate from the
physical storage of data. In a similar way, the actual tables and columns are logical entities
within the physical database. The user who makes a request for data from an Oracle
Database probably doesn't know anything about instances and tablespaces, but does
know about the structure of her data, as implemented with tables and columns. To fully
leverage the power of Oracle, you must understand how the Oracle Database server
implements and uses these logical data structures, the topic of this chapter.
Datatypes
The
datatype
is one of the attributes for a
column
or a variable in a stored procedure. A
datatype describes and limits the type of information stored in a column, and can limit
the operations that you can perform on columns.
You can divide Oracle datatype support into three basic varieties: character datatypes,
numeric datatypes, and datatypes that represent other kinds of data. You can use any of
these datatypes when you create columns in a table, as with this SQL statement:
CREATE TABLE SAMPLE_TABLE(
char_field CHAR(10),
varchar_field VARCHAR2(10),
todays_date DATE)
You also use these datatypes when you define variables as part of a PL/SQL procedure.
