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43 Terms
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Database
They are a collection of organised information so the information can be easily accessed, processes and managed.
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Database Management System (DBMS)
It is a software package designed to define, manipulate, retrieve and manage data in a database
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Entity
It is one type of object.
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Attribute
They describe the entity
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They are also called fields or columns.
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How is a database table organised?
In records, they can also be called rows.
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Data Models
They define how data is connected to each other and how they are processed and stored inside the system.
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Designed to allow easy data input.
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Relational Database
It is a database that has been designed and structured so that the relationships among different entities are identified by shared attributes among different entities.
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Primary Key
It is a unique identifier for a record
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Foreign Key
It is a primary key from a different table used to define the relationship.
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Composite Key
It is a combination of two or more attributes in a table that can be used uniquely to identify each record in the table.
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Relationship
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One-to-One
One entity can relate to the other entity exactly once.
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Relationship
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One-to-Many
One entity can relate to the other entity more than once.
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Relationship
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Many-to-Many
One entity can appeal multiple times in the other entity and vice versa.
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First Normal Form (1NF)
It has no repeating attribute/field or groups of attributes.
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It contains only atomic values.
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Second Normal Form (2NF)
It is in the first normal form and all the *non-key attributes* are fully functional dependant on the primary key.
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This is partially dependant on the primary key only occurs if there is a composite key.
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Third Normal Form (3NF)
It is in second normal form and there is no non-key dependancies or no attributes depending on other attributes.
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Database Normalisation
To reduce data redundancy.
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To maintain data integrity.
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Smaller tables, faster indexing, fast sorting, searching, updating.
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No accidental deleting a record.
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DDL (Data Definition Language)
It is a standard for commands that define the different structures in a database.
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Statements create, modify and remove database objects like tables.
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Inner join
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Client-Server Database Model
It is a centralised database server dealing with many requests from different clients on different PCs
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Advantages of using a Client-Server Database Model
Avoids data inconsistency since only one copy of the database is accessed.
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All updates will be immediately available to all clients.
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Avoid expensive cost of resources required to have multiple copies of a database.
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Access rights and security can be centrally managed.
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Disadvantages of using a Client-Server Database Model
When clients update the same record at the same time, some updates will be lost.
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To prevent data inconsistency, a DBMS uses *record locking* mechanism - whenever a record is being updated, it is being locked until the transaction is completed or cancelled.
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Serialisation
It is a mechanism to ensure there is not transactions overlap in time. A transaction cannot start until the previous one has finished.
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Serialisation Techniques
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Timestamp Ordering
A timestamp is given at the start of each transaction. For two transactions updating the same record, the transaction with the earlier timestamp will be applied first.
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Serialisation Techniques
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Commitment Ordering
Transactions are ordered by their dependencies on each other as well as the time they started.
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If the transaction has dependencies on another, it it blocked until the dependent-upon transactions have finished/committed.
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