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Introduction to SQL Queries

Introduction​

In the realm of databases, SQL (Structured Query Language) plays a pivotal role as it is used to manipulate and retrieve data stored in relational databases. In this tutorial, we'll delve into the basics of SQL queries, focusing on MySQL - a popular open-source relational database management system.

What is an SQL Query?​

A query in SQL is essentially a request made to the database for specific information. You can consider SQL queries as the means by which you interact with a database, retrieving data, inserting new records, updating existing data, or deleting data.

Types of SQL Queries​

SQL queries are generally divided into the following categories:

  1. DDL (Data Definition Language): These queries are used to define or alter the structure of the database objects. Examples include CREATE, ALTER, DROP, and TRUNCATE.

  2. DML (Data Manipulation Language): These queries are used to manipulate the data within the database objects. Examples include SELECT, INSERT, UPDATE, and DELETE.

  3. DCL (Data Control Language): These queries are used to control the visibility of data within the database like granting permissions and roles. Examples include GRANT and REVOKE.

Basic SQL Queries​

Let's look at some examples of basic SQL queries.

SELECT​

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

Example:

SELECT * FROM Employees;

In this example, * is a wildcard character that means "all". So, the query is asking to select all fields for all records in the table 'Employees'.

INSERT INTO​

The INSERT INTO statement is used to insert new records in a table.

Example:

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

This query will insert a new record into the 'Employees' table with the values 'John', 'Doe', and 30 in the fields 'FirstName', 'LastName', and 'Age' respectively.

UPDATE​

The UPDATE statement is used to modify the existing records in a table.

Example:

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

This query will update the 'Age' field to 31 for the record(s) in the 'Employees' table where 'FirstName' is 'John' and 'LastName' is 'Doe'.

DELETE​

The DELETE statement is used to delete existing records in a table.

Example:

DELETE FROM Employees WHERE FirstName = 'John' AND LastName = 'Doe';

This query will delete the record(s) from the 'Employees' table where 'FirstName' is 'John' and 'LastName' is 'Doe'.

Conclusion​

This tutorial introduces the basics of SQL queries. With these foundational elements, you can interact with databases, manipulate data, and retrieve information. The power of SQL lies in its simplicity and robustness, enabling you to manage complex data structures with simple queries. As you become more familiar with SQL, you'll learn more advanced queries and functions that will further enhance your data manipulation capabilities.