Chapter 12 Representing Data Elements By Yue Lu CS257 Spring 2008 Instructor: Dr.Lin

Contents Data Elements and Fields Data Elements and Fields –Representing Relational Database Elements –Representing Objects –Representing Data Elements Records Records –Building Fixed-Length Records –Record Headers –Packing Fixed-Length Records into Blocks

Contents Data Elements and Fields Data Elements and Fields –Representing Relational Database Elements –Representing Objects –Representing Data Elements Records Records –Building Fixed-Length Records –Record Headers –Packing Fixed-Length Records into Blocks

Data Elements and Fields Representing Relational Database Elements Representing Relational Database Elements –Consider the MovieStar relation below declared in SQL CREATE TABLE MovieStar ( CREATE TABLE MovieStar ( name CHAR(30) PRIMARY KEY, name CHAR(30) PRIMARY KEY, address VARCHAR(255), gender CHAR(1), birthdate DATE ); –DBMS has to represent and store the relation

Data Elements and Fields Issues in representing relational DB elements 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

Contents Data Elements and Fields Data Elements and Fields –Representing Relational Database Elements –Representing Objects –Representing Data Elements Records Records –Building Fixed-Length Records –Record Headers –Packing Fixed-Length Records into Blocks

Data Elements and Fields Representing Objects Representing Objects –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

Contents Data Elements and Fields Data Elements and Fields –Representing Relational Database Elements –Representing Objects –Representing Data Elements Records Records –Building Fixed-Length Records –Record Headers –Packing Fixed-Length Records into Blocks

Data Elements and Fields

Representing Data Elements Representing Data Elements –Fixed-Length Character Strings CHAR(n): n bytes 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 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

Data Elements and Fields Representing Data Elements (con.) Representing Data Elements (con.) –Dates and Times Date: 10 bytes (YYYY-MM-DD) Date: 10 bytes (YYYY-MM-DD) Time: 8 bytes (HH:MM:SS) Time: 8 bytes (HH:MM:SS) –May need more bytes to include fraction of a second –Bits –Enumerated Types

Contents Data Elements and Fields Data Elements and Fields –Representing Relational Database Elements –Representing Objects –Representing Data Elements Records Records –Building Fixed-Length Records –Record Headers –Packing Fixed-Length Records into Blocks

Records Building Fixed-Length Records Building Fixed-Length Records –All the fields of the record have a fixed length –E.g. A MovieStar record

Records Building Fixed-Length Records Building Fixed-Length Records –Sometimes, it is more efficient to align fields starting at a multiple of 4

Contents Data Elements and Fields Data Elements and Fields –Representing Relational Database Elements –Representing Objects –Representing Data Elements Records Records –Building Fixed-Length Records –Record Headers –Packing Fixed-Length Records into Blocks

Records Record Headers Record Headers –Pointer to the record schema –Length of the record –Timestamp

Contents Data Elements and Fields Data Elements and Fields –Representing Relational Database Elements –Representing Objects –Representing Data Elements Records Records –Building Fixed-Length Records –Record Headers –Packing Fixed-Length Records into Blocks

Records Packing Fixed-Length Records into Blocks Packing Fixed-Length Records into Blocks

Records Header Header –Links to other blocks –Information about the role played by this block in such a network –Information about which relation the tuples of this block belong to –Directory giving the offset of each record in the block –Block ID –Timestamp

References Database Systems The Complete Book by Hector Garcia- Molina, Jeffrey D.Ullman and Jennifer Widom Fundamentals of Database Systems The Complete Book by Hector Garcia- Molina, Jeffrey D.Ullman and Jennifer Widom Fundamentals of Principles of Database Systems with Internet and Java Applications by Greg Riccardi Principles of Database Systems with Internet and Java Applications by Greg Riccardi Database Systems by Elmasri Navathe Database Systems by Elmasri Navathe 07spring.dir/chap3.pdf 07spring.dir/chap3.pdf