Defining user sessions with SQL gives flexibility and full control over how metrics are defined for unique business needs, allowing analysis of user interactions within a given time frame. A session is typically defined as a group of interactions by one user within that time frame, usually 30 minutes, including events like page visits or actions completed on an app. To define sessions, an event table with user_id and timestamp is required, along with additional information like event type and referrer data. SQL queries using window functions can be used to calculate inactivity times and group events into sessions based on 30-minute intervals of inactivity. By analyzing these sessions, businesses can answer questions about user behavior, such as average session duration and bounce rates.