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A2 1.3.2 - Databases


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First Normal Form
Normalisation of a database following rules of having no repeating attributes or groups of attributes
unique
1NF Rule: each row must be ...
name
1NF Rule: each field must have a unique ...
repeated data
1NF Rule: no columns can have similar or ...
broken down further
1NF Rule: each data item should not be able to be ...
Second Normal Form
Normalisation of a database so that it is in First Normal Form and contains no partial dependencies
partial dependency
when an attribute in a table depends on only one part of the primary key
composite key
Second Normal Form only applies to tables which have a ...
Third Normal Form
Normalisation of a database so that it is in Second Normal Form and contains no 'non-key dependencies'
non-key dependency
when an attribute in a table depends on another attribute in the table which is not (part of) the primary key
data redundancy
data that appears in more than one database table (which can reduce data integrity)
edit
Advantage of Database Normalisation: it is easier to maintain and ...
inconsistencies
Advantage of Database Normalisation: it removes the possibility of having redundant data, helping prevent inefficiencies and ...
does not exist
Advantage of Database Normalisation: it is impossible to use details in one table for another entity that ...
sorting and searching
Advantage of Database Normalisation: it produces smaller tables, allowing for faster ...
storage space
Advantage of Database Normalisation: as data is only stored once in the database, normalised databases save ...
accidental deletion
Advantage of Database Normalisation: it helps to prevent the ... of records
structured query language
a declarative language used for querying and updating tables in a relational database
SELECT
SQL: extracts the data in the requested fields of a specified table
FROM
SQL: declares the table(s) to get data from
WHERE
SQL: specifies the search criteria
ORDER BY
SQL: declares the field that the results should be sorted on
IS NULL
SQL: condition to check whether the field doesn't contain a value
BETWEEN ... AND
SQL: condition to specify a range including the two values that define the limits
IN
SQL: condition that the value must be within a set of values
LIKE
SQL: condition to find values that are similar to the one specified
INNER JOIN
SQL: combines rows from multiple tables based on a common field between them
ON
SQL: specifies the common field that tables should be joined by
%
SQL: wildcard, representing a string of any length
_
SQL: wildcard, representing a single character
COUNT
SQL: returns the number of records that meet the criteria
GROUP BY
SQL: used with count and will return the number of records for every value that appears
CREATE TABLE
SQL: creates a table with the specified name and fields that are defined within ()
ALTER TABLE
SQL: allows the specified table to be modified
ADD
SQL: adds a column to the table when used with the ALTER TABLE statement
DROP COLUMN
SQL: deletes a column from the table when used with the ALTER TABLE statement
MODIFY COLUMN
SQL: allows changes to be made to a column when used with the ALTER TABLE statement
FOREIGN KEY
SQL: creates a column that is a foreign key when creating a table
REFERENCES
SQL: used with the FOREIGN KEY statement to specify what table and field the foreign key links to
INSERT INTO
SQL: used to create a new record in a database table
VALUES
SQL: used with the INSERT INTO statement to specify what values should be in the record
UPDATE
SQL: modifies the values of a record in the database
SET
SQL: used with the UPDATE statement to set the values of columns
DELETE
SQL: deletes a record from a database
transaction
a single logical operation on data in a database
atomicity
a transaction must be processed in its entirety or not at all (ie. it is not possible to only process part of a transaction in event of the database crashing)
consistency
a transaction cannot violate any validation rules for maintaining the integrity of the database (such as the defined referential integrity rules)
isolation
the execution of concurrent transactions should have the same result regardless of the order that they are processed
durability
once a transaction has been committed, it must remain committed
record locking
technique of preventing simultaneous access to objects in a database from causing updates to be lost or data inconsistencies
deadlock
when two users are trying to access two records and lock them in such an order that neither can proceed with the transaction
serialisation
technique to ensure that transactions don't overlap in time and cannot lead to updates being lost
timestamp ordering
technique to ensure that the transaction which began first, will be applied first before the other
commitment ordering
technique that orders transactions in terms of their dependencies on each other
redundancy
technique of having additional hardware / measures in place to prevent the loss of data in the event of a power failure or other disaster