Question

I have two tables – one called customer_records and another called customer_actions.

customer_records has the following schema:

CustomerID (auto increment, primary key)
CustomerName
...etc...

customer_actions has the following schema:

ActionID (auto increment, primary key)
CustomerID (relates to customer_records)
ActionType
ActionTime (UNIX time stamp that the entry was made)
Note (TEXT type)

Every time a user carries out an action on a customer record, an entry is made in customer_actions, and the user is given the opportunity to enter a note. ActionType can be one of a few values (like ‘designatory update’ or ‘added case info’ – can only be one of a list of options).

What I want to be able to do is display a list of records from customer_records where the last ActionType was a certain value.

So far, I’ve searched the net/SO and come up with this monster:

SELECT * FROM (
    SELECT * FROM (
        SELECT * FROM `customer_actions` ORDER BY `EntryID` DESC
    ) list1 GROUP BY `CustomerID`
) list2 WHERE `ActionType`='whatever' LIMIT 0,30

Which is great – it lists each customer ID and their last action. But the query is extremely slow on occasions (note: there are nearly 20,000 records in customer_records). Can anyone offer any tips on how I can sort this monster of a query out or adjust my table to give faster results? I’m using MySQL. Any help is really appreciated, thanks.

Edit: To be clear, I need to see a list of customers who’s last action was ‘whatever’.

Answer 1

To filter customers by their last action, you could use a correlated sub-query…

SELECT
  *
FROM
  customer_records
INNER JOIN
  customer_actions
    ON  customer_actions.CustomerID = customer_records.CustomerID
    AND customer_actions.ActionDate = (
           SELECT
             MAX(ActionDate)
           FROM
             customer_actions  AS lookup
           WHERE
             CustomerID = customer_records.CustomerID
        )
WHERE
  customer_actions.ActionType = 'Whatever'

You may find it more efficient to avoid the correlated sub-query as follows…

SELECT
  *
FROM
  customer_records
INNER JOIN
  (SELECT CustomerID, MAX(ActionDate) AS ActionDate FROM customer_actions GROUP BY CustomerID) AS last_action
    ON customer_records.CustomerID = last_action.CustomerID
INNER JOIN
  customer_actions 
    ON  customer_actions.CustomerID = last_action.CustomerID
    AND customer_actions.ActionDate = last_action.ActionDate
WHERE
  customer_actions.ActionType = 'Whatever'