MYSQL重构

Question

I am newer to setting up MYSQL queries. I have come up with a query that has a sub query, in a sub query to get the desired results. However, I am looking to speed the query up on my database as right now it is taking around 5 seconds to pull the information and do the work. Any help would be appreciated:

SELECT AVG( Total ) AS Average
FROM (

  SELECT Total, ID, MIN( DATE ) 
  FROM (

    SELECT SUM( Amount ) AS Total,  `TransactionDate` AS DATE,  `ClientId` AS ID
    FROM  `transactions` 
    WHERE  `ClientId` <>  ''
    GROUP BY  `InvoiceId` 
    ORDER BY DATE ASC
    ) AS Invoice

  GROUP BY ID
  ) AS AllInvoices

The query needs to find all transactions on a given invoice (unfortunately this is stored as one number in the database already), then group them and get the earliest invoice for each client, then average the totals. Looking to get the average first visit total in the end.

Unfortunately I can't provide all table information, it is imported from a third party and I am not at liberty to fully describe it out. For the little bit that I can share:

Field           Type          NULL  KEY   Default
Id              varchar(900)  NO    MUL   NULL
ClientId        varchar(1000) YES         NULL
TransactionDate datetime      YES         NULL
Amount          decimal(15,4) YES         NULL

The Transaction table currently has 241613 total rows. This could be more or less at any given point in time. These rows don't only contain what a customer paid, but also potentially salary payments and the like included.

However, The only index on table is with the individual transaction IDs themselves, not a value that I am currently pulling.

MYSQL Information:

innodb_version           5.5.43
protocol_version         10
slave_type_conversions
version                  5.5.43-0ubuntu0.14.04.1
version_comment          (Ubuntu)
version_compile_machine  x86_64
version_compile_os       debian-linux-gnu