mysql update one table from another

Question

I have a massive database over over 2.7 million rows. It contains data on uk property prices.

The first Table is called PricePaid And has a column called Price and Loc4.

Now I am trying to get the average for each year grouped by loc4 and update another tabel called PricePaidByCounty.

I have created this SQL statement :

INSERT PricePaidByCounty (County, Avg2013)
SELECT Loc4,
Avg(Price) as AvgPrice2013 FROM PricePaid WHERE Date Like '%2013%'
Group BY Loc4

This works fine for inserting initial row but I want to use an update statement instead as I will need to run this SQL query each month.

Can anyone show me how to change this Insert into an update.

I am doing this as I need to quickly display average house price for each location by year. And the Database is that big I dont want to do this on the flu

Thanks

Answer 1

You can update your table using following query using insert ... on duplicate key update statement ( https://dev.mysql.com/doc/refman/5.0/en/insert-on-duplicate.html ):

INSERT INTO PricePaidByCounty (County, Avg2013)
SELECT Loc4, Avg(Price) as AvgPrice2013 
FROM PricePaid 
WHERE Date Like '%2013%' 
GROUP BY Loc4
ON DUPLICATE KEY UPDATE Avg2013=AvgPrice2013

For this to work you need to make sure that a set of (County, Avg2013) is defined as unique key.

Answer 2

Looks like table PricePaidByCounty needs three columns Country , Year and Average so that while updating by year you can choose to update by particular year. Then the query would be like:

UPDATE PricePaidByCounty ppbc JOIN (SELECT Loc4,
Avg(Price) as AvgPrice FROM PricePaid WHERE Date Like '%2013%'
Group BY Loc4) ap SET ppbc.Average = ap.AvgPrice WHERE ppbc.Country = ap.Loc4 AND ppbc.Year = 2013;

This can be enhanced to group by Loc4 and Year and directly update to PricePaidByCounty . It may run for days as you have multiple records. :D