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Chapter 12 Representing Data Elements By Yue Lu CS257 Spring 2008 Instructor: Dr.Lin Contents Data Elements and Fields – Representing Relational Database Elements – Representing Objects – Representing Data Elements Records – Building Fixed-Length Records – Record Headers – Packing Fixed-Length Records into Blocks Contents Data Elements and Fields – Representing Relational Database Elements – Representing Objects – Representing Data Elements Records – Building Fixed-Length Records – Record Headers – Packing Fixed-Length Records into Blocks Data Elements and Fields Representing Relational Database Elements – Consider the MovieStar relation below declared in SQL CREATE TABLE MovieStar ( 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 – 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 – Representing Relational Database Elements – Representing Objects – Representing Data Elements Records – Building Fixed-Length Records – Record Headers – Packing Fixed-Length Records into Blocks Data Elements and Fields Representing Objects – Objects can have methods or specialpurpose 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 – Representing Relational Database Elements – Representing Objects – Representing Data Elements Records – Building Fixed-Length Records – Record Headers – Packing Fixed-Length Records into Blocks Data Elements and Fields Data Elements and Fields Representing Data Elements – 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 Data Elements and Fields 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 – Enumerated Types Contents Data Elements and Fields – Representing Relational Database Elements – Representing Objects – Representing Data Elements Records – Building Fixed-Length Records – Record Headers – Packing Fixed-Length Records into Blocks 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 – Sometimes, it is more efficient to align fields starting at a multiple of 4 Contents Data Elements and Fields – Representing Relational Database Elements – Representing Objects – Representing Data Elements Records – Building Fixed-Length Records – Record Headers – Packing Fixed-Length Records into Blocks Records Record Headers – Pointer to the record schema – Length of the record – Timestamp Contents Data Elements and Fields – Representing Relational Database Elements – Representing Objects – Representing Data Elements Records – Building Fixed-Length Records – Record Headers – Packing Fixed-Length Records into Blocks Records Packing Fixed-Length Records into Blocks Records 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 GarciaMolina, Jeffrey D.Ullman and Jennifer Widom Fundamentals of Principles of Database Systems with Internet and Java Applications by Greg Riccardi Database Systems by Elmasri Navathe http://dbserver.kaist.ac.kr/~mhkim/cs56007spring.dir/chap3.pdf