How do MySQL Self-Joins Help Find Events That Occur One Day After Another?

Understanding MySQL Self-Joins

Understanding self-joins in MySQL can be daunting, especially if you're encountering them for the first time. Let's dissect the following query to grasp its mechanics:

SELECT event2.id, event2.startdate, event2.price
FROM mm_eventlist_dates event1
JOIN mm_eventlist_dates event2 
ON event2.startdate = date_add(event1.enddate, INTERVAL 1 DAY)
WHERE event1.id = $id

Breaking Down the Query:

FROM and JOIN:

• This query combines data from two identical tables, mm_eventlist_dates, using the JOIN keyword.
• It creates two virtual tables, event1 and event2, representing the same table twice.

WHERE:

• This clause selects a specific record from event1 based on its ID. Let's assume this record represents Event1.

ON:

• This clause defines the relationship between the two virtual tables.
• It compares the start date of event2 with the end date of event1, offset by one day.
• This condition ensures that the start date of event2 should be exactly one day after event1 ends.

SELECT:

• This clause specifies the fields that you want to retrieve from the query.
• In this case, it selects the ID, start date, and price of event2.

How the Query Works:

1. The query first identifies the record from event1 that matches the specified ID passed through the $id variable.
2. For the identified record from event1, it calculates the date that is one day after its end date.
3. The calculated date is then compared against the start date of records in event2.
4. Since event1 and event2 are identical tables, the only matching record in event2 will have a start date that is one day after event1 ends.
5. This matching record from event2 represents the following event (Event2) that occurs one day after Event1.
6. Finally, the query retrieves the ID, start date, and price of Event2 and displays them as the result.

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