MySQL查询以更新列表计数

Question

I have a MySQL problem which I'm struggling to solve.

I have two tables. The first is a table of many thousands of garden items each with a groupid, with many items falling into any given group id. The second table is a summary table, simply listing the groupids and the count of items in each.

I need a query that will regularly update this second table o be run as part of a batch process overnight.

So far I have got this far - the first query selects the groups and counts in the first table and next to them what the new counts should be - I am not sure how to complete it:

SELECT l.listid,l.subscribecount,count(ls.listid) 
FROM `lists` as l,`list_items` as ls 
where l.listid=ls.listid
group by listid;

update lists    
left join list_items on
    lists.listid = list_items.listid
set
    lists.subscribecount = count(list_items.listid);

Any help would be appreciated as I am pretty new to mysql.

Answer 1

You could use the update-join syntax:

UPDATE lists l
JOIN   (SELECT   listid, COUNT(*) AS cnt 
        FROM     list_items 
        GROUP BY listid) li ON l.listid = li.listid
SET    l.subscribecount = cnt