Databases Flashcards
1
Q
What is a database?
A
- A structured persistent store of data. Stored on the secondary storage devices of a computer system
- Provide security, auto backups and users with controlled access to data they need
2
Q
Benefits of databases?
A
-Make it fast to: access data, update data, search for data and present data in a suitable way
2
Q
What are the layers of a Database?
A
- External level: What the user sees. Acts like a filter on data in the database Designed to be useful for a particular job. User friendly UI. Hides unnecessary/restricted data. Allows user to interact with DB
- Conceptual view: Describes structure of DB, entities, attributes, data types, relationships, validation, tables. Hides details of physical storage structures
- Physical: How the data is actually stored in the DB, works with OS and DBMS to store and retrieve data from storage devices
3
Q
What is data security?
A
- Keeping data safe from accidental or deliberate loss and malicious attacks.
- Controls Who has access to the database and what they are permitted to do
4
Q
What are files?
A
- Data stores on a computers secondary storage medium that have names and are stores of binary data
- Can be a program, word processed document, image or music
5
Q
What are serial data files?
A
- Records organised one after another
- Only possible way to storage files on mediums such as tape
- Each data file is as large as the data to be stored
- No empty records = efficient
- Slow as it has to read through each data record until relevant record is found
- If data is modified then the complete altered file has to be re-written back to storage medium. Temp file created and replaces original until process is complete
6
Q
What are sequential data files?
A
- Same as serial but records are stored in order of a designated key field (Eg: alphabetical)
- Records can be found easier
- Data modification still raises same issues as serial
- Not great for data that has no particular order as data has to be sorted often
7
Q
What is indexing?
A
- Speed up searches
- Software searches for the start of the right section (like a book chapter), then directs the search to the right part of the main data store. After that a sequential search can look for the item required
- Records match length of data to be stored
- No wasted space
- Deleting a record will only delete index number, data still remains
- New records are added to end of files and index number is increased
- Eventually DB will have large amounts of wasteful data that is not indexed. Can be solved by rewriting the file and therefore removing any un-indexed data and re-indexing the rest
8
Q
What is an indexed sequential data file?
A
- Used for large Sequential Data Files
- Stored in order of key field AND index
9
Q
What is a random access data file?
A
- Each record is ID with a record number in sequence and each record occupies same space
- Quick access to data as it can calculate position based on record number
- Wasteful of storage as EVERYTHING is stored with same amount of memory
- Each time new file is added it is added to the end and the RN is Incremented
- When records are removed only the data is deleted, leaving an empty record. Overtime there can be many blank records
10
Q
What are fixed length fields?
A
- Suitable amount of space In a file is reserved for data
- Any unused bytes are packed with a character such as a space
- Less space efficient however processing is more straightforward because bytes can be counted to reach desired point. (After set ‘n’ number of bytes you will move onto next field)
- Quick to search
11
Q
What are variable length fields?
A
- Only store as much data as is needed. Eg “Fred’ takes 4 bytes and ‘Jennifer’ takes 8 bytes
- Space efficient
- Processing is harder as finding a particular record requires checking all the preceding entries sequentially
12
Q
What is a flat file database?
A
- Simple databases with one table
- A single spreadsheet is an example
- used for simple data-handling requirements
13
Q
What is a relational database?
A
- Multiple related tables
- Each table represents a single entity (real world object or person)
- Required for the storage of more complex data
- Tables are linked via common attributes to create relationships (Primary keys —> foreign keys)
- Reduces amount of duplicated data
- Entity is anything about which we store data. They have attributes (characteristics eg: student name, ID, subject level)
- Tables consist of Tuples (records)
14
Q
What is a hierarchal database?
A
- Multiple tables
- Tree like structure
- Each child node linked to parent node
15
Q
Structure of relational databases? (Including primary, secondary and foreign keys)
A
- Use fixed length fields
- Each column contains 1 data type
- No rule about order of rows in a table. When order is required indexes are added
- No rule about order of columns
- No two rows can be identical
- One column must be primary key therefore making each row unique
- Primary key is used to link a table to the foreign keys of other tables. Relationships link them together and are produced by having repeated fields across tables (foreign key)
- Secondary keys are not necessarily unique but can be useful for quick searching
