SQL - An Overview

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The key functionalities of SQL include:

1. Querying Data: SQL is primarily known for its ability to query data from databases. This is typically done using the SELECT statement. You can fetch data from one or many tables based on specified conditions.

2. Modifying Data: You can use SQL to insert new data or update existing data within a database. This is done using the INSERT and UPDATE statements. The DELETE statement is used to remove existing data.

3. Creating and Modifying Database Structure: SQL allows you to create new databases and tables using the CREATE DATABASE and CREATE TABLE statements. Additionally, you can modify the structure of an existing table with the ALTER TABLE statement.

4. Data Control: SQL also includes commands for controlling access to the data and commands in a database. This is important for maintaining security and data integrity.

SQL operates through declarative statements, meaning you tell the system what result you want, not how to get there. The database management system figures out the best way to execute the query.

Most relational database management systems (RDBMS) use SQL, including MySQL, Oracle, PostgreSQL, and SQL Server. There are slight variations in SQL syntax and capabilities between these systems, but the basic functionality and commands are similar.

Here are some of the most common SQL operations and some examples of how they're used:

1. SELECT: Used to select data from a database. The data returned is stored in a result table, called the result-set.

   Example:

   SELECT * FROM Employees;

   This will select all data from the "Employees" table.

2. INSERT INTO: Used to insert new data into a database.

   Example:

   INSERT INTO Employees (FirstName, LastName, Age) VALUES ('John', 'Doe', 30);

   This will add a new row to the "Employees" table with the first name 'John', last name 'Doe', and age 30.

3. UPDATE: Used to modify the existing records in a table.

   Example:

   UPDATE Employees SET Age = 31 WHERE FirstName = 'John' AND LastName = 'Doe';

   This will update the age of 'John Doe' in the "Employees" table to 31.

4. DELETE: Used to delete existing records in a table.

   Example:

   This will delete 'John Doe' from the "Employees" table.

5. CREATE DATABASE: Used to create a new database.

   Example:

   This will create a new database named 'Company'.

6. ALTER TABLE: Used to add, delete/drop or modify columns in an existing table.

   Example:

   This will add a new column named 'Email' to the "Employees" table.

7. DROP TABLE: Used to delete a table.

   Example:

   This will delete the "Employees" table from the database.

8. CREATE TABLE: Used to create a new table.

   Example:

   This will create a new table named 'Employees' with columns 'EmployeeID', 'FirstName', 'LastName', and 'Age'.

These are some of the basic SQL commands. SQL also includes many other commands and functions for more complex operations and queries.