How do MySQL self-joins find events following another event by one day?

Understanding MySQL Self-Joins

A recent query explored the concept of self-joins in MySQL. The goal was to retrieve information about an event (Event2) that follows another event (Event1) by one day. While the query successfully executes, further understanding is sought regarding its functionality.

Breaking Down the Query:

The query can be broken down into several distinct parts:

• Data Retrieval:

  • FROM mm_eventlist_dates event1: Selects data from a table named mm_eventlist_dates and assigns the alias event1.
  • JOIN mm_eventlist_dates event2: Performs a self-join on mm_eventlist_dates, creating a second virtual table aliased as event2.

• Join Condition:

  • ON event2.startdate = date_add(event1.enddate, INTERVAL 1 DAY): Specifies the join condition that filters event2 records based on their startdate matching the enddate of event1 plus one day.

• Filtering:

  • WHERE event1.id = $id: Filters the event1 table to retrieve only the record with the specified ID.

How It Works:

1. Record Selection: The WHERE clause first retrieves the specific event (Event1) based on its ID.
2. Date Calculation: The date_add() function calculates the next day after the selected Event1's enddate.
3. Join Filtering: The join condition then matches records in event2 where the startdate matches the calculated date.
4. Data Extraction: Finally, the SELECT clause retrieves three fields from the event2 table: ID, startdate, and price.

Simplified Visualization:

Imagine two separate lists of events (Event1 and Event2) where each event has an ID, start date, and price. The query steps are:

1. Locate Event1 with the specified ID.
2. Calculate the next day after Event1's end date.
3. Identify Event2 events where their start date matches the calculated date.
4. Extract the desired fields from the matching Event2 records.

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