MYSQL多表删除

Question

I have 2 tables setup like this:

Items:
   id (int)
   name (varchar)
   category (int)
   last_update (timestamp)

Categories
   id (int)
   name (varchar)
   tree_id (int)

I want to delete all the records from Items, who's last_update is NOT today, and who's corresponding categories.tree_id is equal to 1. However, I don't want to delete anything from the Categories table, just the Items table. I tried this:

$query = "DELETE FROM items USING categories, items
    WHERE items.category = categories.id
    AND categories.tree_id = 1
    AND items.last_update != '".date('Y-m-d')."'";

However this just seems to delete EVERY record who's tree_id is 1. It should keep items with a tree_id of 1, as long as their last_update field is today

What am I missing?

Answer 1

You say that last_update contains time-stamps - I assume UNIX time-stamp. You can never find a record with the same time-stamp as when the stamp was executed and you can not compare a time-stamp with formatted date, they'll never correspond. So you need to store the data in last_update column in a date(Y-m-d) format so that compare those which are not equal.