MySQL：可变行最大最小查询？

Question

I have a MySql table containing stock quotes (stock_symbol, quote_date, open_price, high_price, low_price, close_price) and need to find out various max and min values: Max High/Min Low over 10/20/245/252 days.

I know how to do this using a SELECT query for each period:

SELECT max(high_price), min(low_price) FROM mytable WHERE stock_symbol = 'GOOG' ORDER BY quote_date DESC LIMIT 10;

Can it be done using a single SELECT statement without making the query too intense for the database server? Would a single SELECT statement perform better than multiple SELECT statements?

Any thoughts/help would be much appreciated. Thanks!

Answer 1

Do it preferably in one statement, but the problem is only on the indexes : be sure to have an index on all columns for which you ask the min and the max or that you add in the where clause : hight_price, low_price and stock_symbol.

With indexes, this request will be instantaneous, even for very big tables and without your LIMIT 10

Don't put an ORDER BY in this query : it's useless as the min and max are the same independently of the order you look at the set.

Your query should be simply

SELECT max(high_price), min(low_price) FROM mytable WHERE stock_symbol = 'GOOG'

How to create an index : http://dev.mysql.com/doc/refman/5.0/en/create-index.html