SQL injection usually occurs when you ask a user for input, like their username/userid, but the user gives ("injects") you an SQL statement that you will unknowingly run on your database. For example:
Look at the following example, which creates a SELECT statement by adding a variable (txtUserId) to a select string. The variable is fetched from user input (getRequestString):
txtUserId = getRequestString("UserId");
txtSQL = "SELECT * FROM Users WHERE UserId = " + txtUserId;
If user enter something like this: "100 OR 1=1" then the SzQL statement will look like this:
SELECT * FROM Users WHERE UserId = 100 OR 1=1;
The SQL above is valid and will return ALL rows from the "Users" table, since OR 1=1 is always TRUE. A hacker might get access to all the user names and passwords in this database.

A DoS (Denial of Service) attack is a type of cyberattack that aims to disrupt the normal functioning of a server, service, or network by overwhelming it with a large amount of traffic or requests. A DoS attack typically uses a single computer or device to launch the attack, sending TCP (Transmission Control Protocol) or UDP (User Datagram Protocol) packets to the target server. TCP and UDP are two common protocols used to send data over the internet. TCP packets require a connection to be established between the sender and the receiver, and ensure that the data is delivered reliably and in order. UDP packets do not require a connection, and do not guarantee the delivery or order of the data. Both TCP and UDP packets can be used to flood a server with requests, consuming its resources and bandwidth, and preventing legitimate users from accessing the service.
A DDoS (Distributed Denial of Service) attack is a type of DoS attack that uses multiple computers or devices to launch the attack, creating a large network of attackers that can generate more traffic or requests than a single source. A DDoS attack often involves a botnet, which is a network of compromised computers or devices that are controlled by a malicious actor, usually through malware or hacking. The botnet can send TCP or UDP packets to the target server from different locations and IP addresses, making it harder to trace and block the attack. A DDoS attack can also target multiple servers or services that are distributed over a LAN (Local Area Network), such as a web hosting service or a cloud computing platform, affecting the availability and performance of the entire network.
The main difference between a DoS attack and a DDoS attack is the number and diversity of the sources that are involved in the attack. A DoS attack comes from a single source, while a DDoS attack comes from multiple sources. This makes a DDoS attack more powerful, faster, and harder to stop than a DoS attack.
References:
Implementing and Operating Cisco Security Core Technologies (SCOR) v1.0, Module 1: Malware Threats, Lesson 2: Identifying Network Attacks, Topic: DoS and DDoS Attacks DoS Attack vs. DDoS Attack: Key Differences? | Fortinet What's the Difference Between a DOS and DDoS Attack? - How-To Geek