Relational Databases

Why Databases As programs get larger, data becomes more important Data is entities and relationships Real-world processes involve complex data interaction

Structure of a Database Record – one discreet entity Field – one piece of data Table – series of like records

Record Represents some entity Consists of many details All details refer to same record

Field Specific data elements Has a name, length, type, value Sometimes length is assumed Each record has same field definitions, but different values

Table A series of records following the same definition All records should describe same kind of thing Each record is a different instance of that thing

Database A series of tables each defining different entities The relationships between those entities Ancillary files, indices, views

Primary Keys A primary key is a field or group of fields that uniquely identifies a record. A primary key comprised of two or more fields is called a composite primary key. Every table must have a primary key! The most important key in a table: Uniquely identifies a specific record throughout a database Identifies a specific table throughout the database Enforces table-level integrity Helps to establish relationships between tables

Foreign Keys A foreign key is important when establishing relationships between tables. To create a foreign key, you would take a primary key from one table and incorporate it in a second table. In the second table, the key becomes a foreign key. Foreign keys enforce relationship-level integrity – values in one table's foreign key field must match exactly with the corresponding values of a second table's primary key field.

Table Relationships We categorize table relationships in the Relational Model as follows: One-to-One (1:1) One-to-Many (1:N) Many-to-Many (N:N) To establish a relationship between tables, we need to match values of a shared field.

One-To-One Relationships (1:1) A record in one table (a parent table) is related to one and only one record in a second table (a child table). A record in a second table (the child table) is related to one and only one record in the first table (the parent table). We create a 1:1 relationship by copying the primary key of a parent table into a child table, where it becomes a foreign key. This type of relationship is unique because both tables share the same primary key. The primary key in the child table serves both as that table's primary key and a foreign key .

One-To-Many Relationships (1:N) A record in one table (a parent table) can be related to many records in a second table (a child table). A single record in the child table is related to one and only one record in the parent table. We create a 1:N relationship by copying the primary key of a parent table into a child table, where it becomes a foreign key. This type of relationship is the most common type of relationship in the relational database model.

Many-To-Many Relationships (N:N) A record in one table can be related to many records in a second table. A single record in the second table can be related to many records in the parent table. We create a N:N relationship by copying the primary keys of a each table into a third table, called a linking (associative) table. Together, the copied keys form a composite primary key. Individually, they serve as foreign keys for the other table.

Example of N:N Relationship

SQL Structured Query Language Uniform system for defining, querying data Used by many programming languages and db tools