Relational Databases :

A relational database at its simplest is a set of tables used for storing data. Each table has a unique name and may relate to one or more other tables in the database through common values.

Tables :

A table in a database is a collection of rows and columns. Tables are also known as entities or relations.

Rows :

A row contains data pertaining to a single item or record in a table. Rows are also known as records or tuples.

Columns :

A column contains data representing a specific characteristic of the records in the table. Columns are also known as fields or attributes.

Relationships :

A relationship is a link between two tables (i.e, relations). Relationships make it possible to find data in one table that pertains to a specific record in another table.

Datatypes :

Each of a table's columns has a defined datatype that specifies the type of data that can exist in that column. For example, the FirstName column might be defined as varchar(20), indicating that it can contain a string of up to 20 characters. Unfortunately, datatypes vary widely between databases.

Primary Keys :

Most tables have a column or group of columns that can be used to identify records. For example, an Employees table might have a column called EmployeeID that is unique for every row. This makes it easy to keep track of a record over time and to associate a record with records in other tables.

Foreign Keys :

Foreign key columns are columns that link to primary key columns in other tables, thereby creating a relationship. For example, the Customers table might have a foreign key column called SalesRep that links to EmployeeID, the primary key in the Employees table.

Relational Database Management System :

A Relational Database Management System (RDBMS), commonly (but incorrectly) called a database, is software for creating, manipulating, and administering a database. For simplicity, we will often refer to RDBMSs as databases.

Popular Databases :

ORACLE
SQL SERVER
DB2

Popular Open Source Databases :

MySQL
PostgreSQL

SQL Statements :

Database Manipulation Language (DML)

DML statements are used to work with data in an existing database. The most common DML statements are:

SELECT
INSERT
UPDATE
DELETE

Database Definition Language (DDL)

DDL statements are used to structure objects in a database. The most common DDL statements are:
CREATE
ALTER
DROP

Database Control Language (DCL)

DCL statements are used for database administration. The most common DCL statements are:

GRANT
DENY (SQL Server Only)
REVOKE