Understanding MySQL Self-Joins

Self-joins, a technique used in SQL databases, allow you to query a table against itself. In this case, we'll delve into a MySQL self-join to find events with start dates following another event by a specified number of days.

Query Breakdown:

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;

1. Data Extraction (FROM)

FROM mm_eventlist_dates event1
JOIN mm_eventlist_dates event2 

This part extracts data from two tables, mm_eventlist_dates, represented as event1 and event2, for self-joining.

2. Join Condition (ON)

ON event2.startdate = date_add(event1.enddate, INTERVAL 1 DAY)

This condition defines the join between the two tables. It specifies that event2's start date must be equal to one day after event1's end date.

3. Filtering (WHERE)

WHERE event1.id=$id

This filter limits the results to events that have an ID matching the specified $id.

4. Field Selection (SELECT)

SELECT event2.id, event2.startdate, event2.price

This part specifies which fields from the event2 table should be included in the result set: ID, start date, and price.

How it Works:

1. The query starts by extracting two sets of records from the mm_eventlist_dates table.
2. The join condition narrows down the result by filtering event2 records whose start dates match one day after the end dates of event1 records.
3. The filter applies the user-specified $id to further refine the event1 records.
4. The results returned include the desired fields from the event2 table, which represent events that follow event1 by one day.