* - represents multiple function in a database
relational operator: > < = => <=
logical operators: and or not
order by :
SQL Syntax:
Define: (162-169)
Data independance: type of data transperancy that matters for a centralised DBMS. Seperation of data from the programs that manipulate data.
concepts of data independace: data is independant from system
Direct and Sequential access: Direct : refers to the ability to go to any data item in any order. Once the location of the required data is known then that data can be read or written directly without accessing or affecting any other data.
e.g. hardisks, usb's
Sequential: means the data must be stored and retrieved in a linear sequence. the tiemt aken to locate data makes sequential storage unsuitable for most application apart from backup
e.g. audio/video tape
On-line Storage: available immediately to connected computers. includes hard disks within a single computer and also storage devices accessed via a network or even over the internet. usually in the form of hard disk drivers. online storage over the net is becomming more common.
Off-line Storage: refers to data stored such that it cannot be accessed until the storage media is mounted into a driver. eg. magnetic tape, optical media such as CD's and DVD's and USB's
Magnetic Storage: provides large storage capacity and in the case of hard disks, it allowd for direct access at high speed for both storing and retrieving process.
- head moves fractions of seconds above the plate
e.g hard disks, tapes, floppy disks,
*Basic understanding of storage media: magnetic, optical, hard disk, RAID and tape
Hardware - Storage.
Hard Disks: store data magnetically on precision aluminium of glass platters. the platters have a layer of hard magnetic material into which the magnetic data is stored. each platter is double sided, so two read/write heads are required for each platter contained within the drive's casing.
RAID: utilises multiple hard disk drives together with a RAID controller. it controller manages the data flowing between the hard disks and the attached computer; the attached computer just sees teh RAID device as a normal single hard disk. stirping data across multiple drives improves speed.
RAID level 1: mirroring data
RADI level 5: parady data seperated into multiple disk drives
Blu Ray discs: have a larger storage capacity because the frequency of light is in the blue region, and not the red region like in other DVDs, although they are the same size.
The higher the frequency of the laser, the more depths the data can be packed on the surface of the disk. Data can be more densely packed on the surface of the disk
1. ProductID of products costing more than $10:
Select.InvoiceProducts.ProductID,InvoiceProducts.InvoiceCosts
From.InvoiceProducts
Where.InvoiceProducts.InvoiceCosts>10
Orderby.
2. Invoice numbers of invoices placed on 09/06/06
Select.
From.
Where.
Orderby.
