📘 Day 94: Data Query Language (DQL)

Welcome to Day 94! Today, we'll focus on the Data Query Language (DQL), which is used to retrieve data from a database. While DQL technically only consists of the SELECT statement, it's the most commonly used part of SQL for data analysis.

The SELECT Statement

The SELECT statement is used to query the database and retrieve data that matches criteria that you specify.

Basic Syntax

SELECT column1, column2, ...
FROM table_name;

WHERE Clause

The WHERE clause is used to filter records.

SELECT column1, column2, ...
FROM table_name
WHERE condition;

ORDER BY Clause

The ORDER BY keyword is used to sort the result-set in ascending or descending order.

SELECT column1, column2, ...
FROM table_name
ORDER BY column1, column2, ... ASC|DESC;

GROUP BY Clause

The GROUP BY statement groups rows that have the same values into summary rows, like "find the number of customers in each country". The GROUP BY statement is often used with aggregate functions (COUNT, MAX, MIN, SUM, AVG) to group the result-set by one or more columns.

SELECT COUNT(CustomerID), Country
FROM Customers
GROUP BY Country;

HAVING Clause

The HAVING clause was added to SQL because the WHERE keyword could not be used with aggregate functions.

SELECT COUNT(CustomerID), Country
FROM Customers
GROUP BY Country
HAVING COUNT(CustomerID) > 5;

💻 Exercises: Day 94

Please see the exercises.sql file for today's exercises.

Previous: Day 93 – Day 93: Data Manipulation Language (DML) • Next: Day 95 – Day 95: Joins

You are on lesson 94 of 108.