How does a MySQL self-join query retrieve data related to an event's end date?

Understanding the Mechanism of MySQL Self-Join Queries

Self-join queries involve joining two instances of the same table to compare or retrieve related data. Let's delve into the specifics of the SQL query you presented:

``
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
``

Diving into the Parts:

• FROM: Specifies the tables to be joined, in this case, both are instances of 'mm_eventlist_dates' tables. We'll refer to them as 'event1' and 'event2' to avoid confusion.
• JOIN: Links the rows from 'event1' to 'event2' based on the specified condition.

• ON: Defines the join condition, which in this query is:

  • event2.startdate = date_add(event1.enddate, INTERVAL 1 DAY)
  • This condition matches rows from 'event2' whose start dates occur one day after the end dates of rows in 'event1'.
• WHERE: Filters the results to only include rows where 'event1.id' matches the specified $id.

How It Works Step by Step:

1. The query starts by identifying the row in 'event1' that matches the given $id parameter.
2. For each matching row in 'event1', it calculates the end date plus one day using the date_add() function, representing the start date of the related row in 'event2'.
3. The 'event1' row is then joined to rows in 'event2' where the start dates match the calculated date from step 2.
4. Finally, the query selects specific columns (ID, start date, and price) from the matched rows in 'event2'.

Example Scenario:

Consider a table with the following Event data:

ID	Start Date	End Date	Price
1	2023-03-01	2023-03-10	0
2	2023-03-15	2023-03-22	0
3	2023-04-01	2023-04-08	0

If you run the query with $id set to 1, it will retrieve the data for Event 2, which starts one day after the end date of Event 1:

ID	Start Date	Price
2	2023-03-15	0

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