MD (multidimensional) structures involve 1, 2, 3, 4, or more dimensions, whereas relational structures involve 2 dimensional tables and columns. In MD structures, a particular subject (called a cube) is looked at from different perspectives (called dimensions). Each dimension may have a hierarchy of aggregates. Each step in the hierarchy is called a level. The upper levels aggregate lower level data, while lower levels segregate upper level data. A chunk of data within a level is a called a member. EG: In the 2000 Wally Sales cube, what were the sales for Time (1999, May, 15) at Store (IL, Chicago, 60647, Will E. Coyote Goods) for Product (Acme, rockets, A1 Turbo)?
Data is placed into an MD structure by an ETL (Extract, Transform, Load) Process.
Here are the common MD structures:
Once you have a MD structure it needs to under go MD Analysis. OLAP (On-Line Analytical Processing) is the presentation of data in MD structures, whereas OLTP (On-Line Transactional Processing) is the use and presentation of data in most relational structures. Data mining is sort of an extension of OLAP that predicts and trends data, whereas plain old OLAP looks at data as is.
MDX (MD Expressions) are used to query a MD structure, just as SQL is used to query a relational structure.
ADO MD (ActiveX Data Objects MD) is used to programmatically access a MD structure and the query results, just as ADO is used to programmatically access a relational structure and the query results.
A typical query to a MD structure yields a cell set with 2-4 axes, whereas a query to a relational structure yields a result set with 2 axes, i.e. a 2D table. An axis may consist of multiple dimensions, each of which may be drilled down to specified various levels. A position may be specified along each axis by specifying the particular dimensions and members along an axis for a cell. A cell is the intersection of the axes as denoted by its position. Each cell can yield particular values called measures. In addition to the primary data of each measure, a cell has other data including a formatted version of the data, and the ordinal value of the cell.
Page Modified: (Hand noted: 2007-10-11 23:40:48Z) (Auto noted: 2011-06-01 16:22:50Z)