Tables

You will see the Relational Database as a collection of one or more tables.

A table:
> Is a two dimensional grid of rows and columns.
> Holds data as a value at each row and column intersection.
> Has at least one column and zero or more rows. (The table is empty if it has no rows.)
> Has a unique name within the Relational Database.

An RDBMS uses two types of tables:
> User tables contain user-defined and maintained data.
> System tables contain data about the tables defined by the users and is called the system catalog. This includes table definitions, security, and performance statistics.

Columns

> Each column reflects a specific property or attribute of the table’s entity.
> Each column has a unique name within its table to signify this property or attribute. However, this same name may also be used in other tables.
> Each column entry contains a single data value.
> The order of the columns from left to right is unimportant.
> Each column has a constraint on its values called the domain. This domain limits the column values to characters and numeric values (names, dates, times, etc.) with other specifications, such as length and null values. A null value means that there is no value, which is not the same as a blank value.

Rows

> Each row reflects a fact about a table’s entity. If this is an employee table, then each row is for a different employee.
> Each column in the row contains a value or a null.
> No two rows of a table can be identical, because each row in a table is uniquely identified by its primary key
> The order of the rows, top to bottom, is unimportant.

E.F. Codd conceived the relational database in 1970. He worked at IBM and published “Codd’s 12 Rules.” A small start-up company released the first RDBMS and called it ORACLE. Then IBM published their version as SEQUEL-XRM and it later became DB2. A Relational DataBase Management System (RDBMS) is a database divided into logical units called tables, where the data in the two-dimensional tables are related to one another within the database. These tables are largely independent, which gives relational databases their flexibility. You can add, change, or delete data in one table without affecting other tables, provided that the changed table is not a parent of any other table. (Parent-child tables are discussed later.)
SQL operators are used to select rows or columns of data from the table, producing a new, temporary result table. This language differs considerably from previous DBMS languages, because it returns one or many rows (or records) with each query. We will show you later how to handle the individual rows. Next we will see how easy SQL is to use!

NETWORK databases were created first. They have very little redundancy but have structural complexity.

HIERARCHICAL databases have a parent/child hierarchy that is faster than network databases, but it suffers from structural inflexibility, redundancy problems, and broken pointers. As it is an older DBMS, it is called a legacy database.

RELATIONAL databases are almost exclusively the database of choice today. Many businesses have converted to Relational DataBase Management Systems (RDBMS) for performance and ease of maintenance.