Writing Basic SELECT Queries

Tutorial 2 of 5

Introduction

The goal of this tutorial is to introduce you to SQL's SELECT statement, a fundamental command that allows you to retrieve data from a database. By the end of this tutorial, you will understand how to write and implement basic SELECT queries.

The prerequisites for this tutorial are a basic understanding of databases and some familiarity with SQL syntax. If you're a complete beginner, don't worry! We'll take things step by step.

Step-by-Step Guide

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

Here's the basic syntax:

SELECT column1, column2, ...
FROM table_name;

You can select one or more columns from the table. If you want to select all columns, you can use the * symbol:

SELECT * FROM table_name;

Best Practices

Keep your column selection as narrow as possible. This will make your queries run faster and reduce the load on the database server.
Always use the ; at the end of your SQL commands. It's not always necessary, but it is good practice and can prevent potential errors.

Code Examples

Let's look at a practical example. Assume we have a table Employees with the following data:

EmployeeId	FirstName	LastName	Department
1	John	Doe	HR
2	Jane	Smith	Marketing
3	Mike	Johnson	Sales

-- This query will select all columns
SELECT * FROM Employees;

Output:

EmployeeId	FirstName	LastName	Department
1	John	Doe	HR
2	Jane	Smith	Marketing
3	Mike	Johnson	Sales

-- This query will select only FirstName and LastName columns
SELECT FirstName, LastName FROM Employees;

Output:

FirstName	LastName
John	Doe
Jane	Smith
Mike	Johnson

Summary

In this tutorial, we have learned the basics of the SELECT statement in SQL, including how to select specific columns or all columns from a table. The next steps would be to learn more about SQL clauses that can be used with SELECT, such as WHERE, ORDER BY, GROUP BY, and JOIN.

Practice Exercises

Exercise: Write a SELECT query to retrieve only the Department column from the Employees table.

Solution:

sql SELECT Department FROM Employees;
This query will return only the Department column from the Employees table.

Exercise: Write a SELECT query to retrieve all columns from the Employees table where the EmployeeId is 2.