Contents
Basic SQL Concepts
Definition: A database is a collection of organized data that can be easily accessed, managed, and updated. It stores data in a structured format, typically in tables.
Example :
CREATE DATABASE myDatabase;
Explanation: This command creates a new database named myDatabase. Creating a database is the first step in organizing and storing data which can then be managed using SQL.
Definition: A table is a collection of related data entries consisting of rows and columns. Each table in a database holds data about a specific subject.
Example :
CREATE TABLE Customers (
CustomerID int,
Name varchar(255),
Address varchar(255)
);
Explanation: This statement creates a table called Customers with columns for CustomerID, Name, and Address. Tables are fundamental for organizing data into specific formats and categories, making it easier to retrieve and manage.
Definition: A schema is a logical container for database objects like tables, views, and procedures.
Example :
CREATE SCHEMA Sales;
Explanation: This command creates a schema named Sales. Schemas help in organizing and securing database objects because they allow you to group related objects under a single name.
Definition: Rows represent individual records in a table, while columns represent the attributes of the records.
Example :
SELECT Name, Address FROM Customers;
Explanation: This query retrieves the Name and Address for each record in the Customers table. Columns help define the data structure of a table, and rows contain the actual data.
Definition: A primary key is a column (or combination of columns) that uniquely identifies each row in a table..
Example :
CREATE TABLE Orders (
OrderID int PRIMARY KEY,
OrderDate date,
CustomerID int
);
Explanation: This creates an Orders table where OrderID serves as the primary key. Primary keys ensure each record within a table can be uniquely identified.
Definition: A foreign key is a column that creates a relationship between two tables by referencing the primary key of another table.
Example :
CREATE TABLE Orders (
OrderID int PRIMARY KEY,
OrderDate date,
CustomerID int,
FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID)
);
Explanation: This statement sets CustomerID as a foreign key in the Orders table that references the CustomerID primary key in the Customers table. Foreign keys are crucial for maintaining referential integrity and representing relationships between data.