mysql limit inside group?

Question

I want to limit the size of records inside a group, and here is my trial, how to do it right?

mysql> select * from accounts limit 5 group by type;
         


        

Answer 1

The point of an aggregate function (and the GROUP BY it requires) is to turn many rows into one row. So if you really just want the top 5 savings accounts and the top 5 chequing accounts and the top 5 USD accounts etc., what you need is more like this:

criteria: top 5 of particular account type by account_balance

SELECT account_type, account_balance FROM accounts WHERE account_type='savings' 
   ORDER BY account_balance DESC LIMIT 5
UNION
SELECT account_type, account_balance FROM accounts WHERE account_type='chequing' 
   ORDER BY account_balance DESC LIMIT 5
UNION
SELECT account_type, account_balance FROM accounts WHERE account_type='USD' 
   ORDER BY account_balance DESC LIMIT 5;

It's not pretty, but if you construct the SQL with a script then subbing in the account_types and concatenating together a query is straightforward.