Lesson 3: SQL Queries

Queries

What is SQL?

SQL (Structured Query Language) is a programming language used to communicate with data stored in a relational database management system. SQL syntax is similar to the English language, which makes it relatively easy to write, read, and interpret.

Queries

A query in itself is just a statement which declares what data we are looking for, where to find it in the database, and optionally, how to transform it before it is returned.

Getting data from a query

SELECT 
column_1
, column_2
, column_3
FROM table;

SELECT *
FROM table;

SELECT 
column_1 AS first_column
, column_2 AS second_column
,  column_3 third_column
FROM table;

Filtering data with a WHERE clause

To refine the data returned by a query, use the WHERE clause to specify conditions.

SELECT 
column_1, column_2
FROM table
WHERE condition;

Example:

SELECT 
name, age
FROM employees
WHERE age > 30;

This query selects the name and age columns from the employees table, returning only rows where the age is greater than 30.