Database Tables, Columns, and Rows
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.



