Query efficiency (multiple selects) - mysql

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'

Answer 2

I'm not sure if I understand the requirements but it looks to me like a JOIN would be enough for that.
SELECT cr.CustomerID, cr.CustomerName, ...
FROM customer_records cr
INNER JOIN customer_actions ca ON ca.CustomerID = cr.CustomerID
WHERE `ActionType` = 'whatever'
ORDER BY
ca.EntryID
Note that 20.000 records should not pose a performance problem

Answer 3

Please note that I've adapted Lieven's answer (I made a separate post as this was too long for a comment). Any credit for the solution itself goes to him, I'm just trying to show you some key points for improving performance.
If speed is a concern then the following should give you some suggestions for improving it:
select top 100 -- Change as required
cr.CustomerID ,
cr.CustomerName,
cr.MoreDetail1,
cr.Etc
from customer_records cr
inner join customer_actions ca
on ca.CustomerID = cr.CustomerID
where ca.ActionType = 'x'
order by cr.CustomerID
A few notes:
In some cases I find left outer joins to be faster then inner joins - It would be worth measuring performance for both for this query
Avoid returning * wherever possible
You don't have to reference 'cr.x' in the initial select but it's a good habit to get into for when you start working on large queries that can have multiple joins in them (this will make a lot of sense once you start doing this
When using joins always join on a primary key

Answer 4

Maybe I'm missing something but what's wrong with a simple join and a where clause?
Select ActionType, ActionTime, Note
FROM Customer_Records CR
INNER JOIN customer_Actions CA
ON CR.CustomerID = CA.CustomerID
Where ActionType = 'added case info'