Flashcards by TS P, updated more than 1 year ago
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Advanced Level (A- Level) Computer Science (Databases) Flashcards on Database, created by TS P on 01/01/2016.

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Drawbacks of file based databases? -Data redundancy -Data inconsistency -Data inconsistency - when the system's applications find it difficult to make updates/retrievals of data from multiple files -Data integrity - it is difficult to alter system applications to implement new constraints, and the application programs require that the input/stored data to meet certain consistency criterias -Data security- there are issues with accessing level privileges as well as implementing these constraints can be difficult -Management of concurrency access - on File based systems, files are locked when a user is using it
How do RDBMS attempt to correct file based systems' weaknesses -Unitary storage in one software -Reduced data inconsistency due to data minimisation -Links can be used to refer to certain tables instead of repeating that data -Reduced data duplications -> reduced quantity of data stored -> faster database access speed -Concurrency management ()
What is a logical schema and what is it intended for? A diagrammatic illustration of the structure of a database's model, showing how items such as tables, described accordingly to the database MS' specifications,
How do data dictionaries in DBMS help address data management issues found in file based systems? Data dictionaries contains: -detailed description of each piece of data -relationships between data -Access levels -Rules for Validations -Protocols for recovering data -A map to distinguish between logics and physics - intended for storing -A log of transactions for monitoring
What are developer Interfaces and Query processors and why are they used? Developer Interface: Allows graphical manipulation of database MS' features so it is more convenient for non-developers Query Processor: handles high level language queries by compiling or interpreting. It parses (syntactically analyse), optimises and validates
What is referential integrity, secondary key and indexing? Prevention of entrance of inconsistent data Secondary key: candidate keys not chosen to be PKs Indexing: the duplication of various columns of data, can be low level disk block addressing or complete duplications, that allows searches, lookups and queries to be performed more quickly.
Criterias for 1NF, 2NF and 3NF? 1NF: data atomicity, no repeated group of attributes 2NF: 1NF and no partial dependency 3NF: 2NF and no non-key dependency
What's the difference between Data Definition Language (DDL) and Data Manipulation Language (DML). DDL: used to define the overall database structure (creation, modification), using commands like ALTER, CREATE DML: Used for data maintenance, with commands like SELECT
Create database syntax? CREATE DATABASE <dbname>
Create table syntax? CREATE TABLE <table name> ( <field name> <field type(size)>; ... PRIMARY KEY (<field name>))
Changing table definition syntax? ALTER TABLE <table name> ADD <field name> <field type(size)> ALTER TABLE <table name> DROP COLUMN <field name> ALTER TABLE <table name> ALTER COLUMN <column name> <new file type>
Add a primary key after table definition? ALTER TABLE <table name> ADD PRIMARY KEY (<field name>) For non-null constraint addition of pk where pk is previously null: ALTER TABLE <table name> ADD CONSTRAINT <field name1> PRIMARY KEY (<fieldname2>,<fieldname3>)
Group by functions? Are functions that group the results by a column GROUP BY (column1,column2,...)
Syntax for inner join? Selects all fields from both tables as long as there is one field that's the same on both table SELECT column_name(s) FROM table1 INNER JOIN table2 ON table1.column1=table2.column2;
what do the fields after the SELECT clause represent? fields that will be shown in the result.
INSERT INTO syntax INSERT INTO tablename (column1,column2,column3,...) VALUES (value1,value2,value3,...);
DELETE FROM syntax? DELETE FROM table_name WHERE some_column=criteria;
UPDATE syntax? UPDATE table_name SET column1=value1,column2=value2,... WHERE some_column=criteria;
How do DBMS avoid data clashes Record locks - locking a record as soon as it is modified, it can only be further modified by the same/different user if acknowledged by the first user that that data needs editing -Serialisation - scheduling transactions (e.g. reading or writing) of data in a serial order -Timestamps - each transaction has a timestamp that indicates when that record was last transacted, and is used by the DBMS to determine whether or not the data should be transacted Commitment Ordering - looks at the priority and/or the impact of a transaction on a database and makes a decision on when to permit that transaction
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