Transcript 257_101_ch12

Chapter 12
Representing Data Elements
• CS257 Spring 2008
• Instructor: Dr.Lin
• Jindou Jiao
Contents
• Data Elements and Fields
– Representing Relational Database Elements
– Representing Objects
– Representing Data Elements
• Attributes need to be represented by
sequences of bytes, called ”field”.
• Fields are put together called “records”.
• Records need to be stored in physical blocks.
• A collection of records form a relation.
Representing Relational Database Elements
• Relational database refers to a database’s data and
schema.
• DBMS has to represent and store the relation
described by the declaration.
– Consider the MovieStar relation below declared in SQL
CREATE TABLE MovieStar (
name CHAR(30) PRIMARY KEY,
address VARCHAR(255),
gender CHAR(1),
birthdate DATE
);
• Issues in representing relational DB
elements
– How to represent SQL datatypes as fields
– How to represent tuples as records
– How to represent collections or records in
blocks of memory
– How to represent and store relations as
collections of blocks
– How to cope with record sizes
– What if the size of a record changes due to
updating
Representing Objects
• Object is a tuple, and its instance variables are
attributes.
• Objects can have methods or special-purpose
functions associated with them.
• Objects may have an object identifier(OID), which
is an address that refers uniquely to that object.
Pointers are used to related objects with each other.
Representing Data Elements
• Datatypes are represented in attributes of a record
• Fixed-Length Character Strings
– CHAR(n): n bytes
• Pad characters are used if the actual value is shorter than n
• Variable-Length Character Strings
– VARCHAR(n): n+1 bytes
• Length plus content: # of bytes in the string + characters of the
string
• Null-terminated string: characters of the string + “null”
character
Representing Data Elements (con.)
• Dates and Times
– Date: 10 bytes (YYYY-MM-DD)
– Time: 8 bytes (HH:MM:SS)
– May need more bytes to include fraction of a
second
• Bits
– Store a sequence of bits
• Enumerated Type