MySQL - UPDATE query based on SELECT Query

Question

I need to check (from the same table) if there is an association between two events based on date-time.

One set of data will contain the ending date-time of certain

Answer 1

I found this question in looking for my own solution to a very complex join. This is an alternative solution, to a more complex version of the problem, which I thought might be useful.

I needed to populate the product_id field in the activities table, where activities are numbered in a unit, and units are numbered in a level (identified using a string ??N), such that one can identify activities using an SKU ie L1U1A1. Those SKUs are then stored in a different table.

I identified the following to get a list of activity_id vs product_id:-

SELECT a.activity_id, w.product_id 
  FROM activities a 
  JOIN units USING(unit_id) 
  JOIN product_types USING(product_type_id) 
  JOIN web_products w 
    ON sku=CONCAT('L',SUBSTR(product_type_code,3), 'U',unit_index, 'A',activity_index)

I found that that was too complex to incorporate into a SELECT within mysql, so I created a temporary table, and joined that with the update statement:-

CREATE TEMPORARY TABLE activity_product_ids AS ();

UPDATE activities a
  JOIN activity_product_ids b
    ON a.activity_id=b.activity_id 
  SET a.product_id=b.product_id;

I hope someone finds this useful