I recently posted this on my personal blog, however it's quite a useful tip so I thought I'd post it here too. Sometimes you may want to perform a mass update on a table based on the contents of another table. The first thing you may think of is using a cursor to scroll through Table A and update Table B line by line. This is extremely slow. However, there is another way.

The way to do it is to join the two tables together then perform a self-update. I'll called the first table "Product" and the second table "UpdatedProduct".

UPDATE Product
    SET Price = NewPrice
    FROM Product
INNER JOIN UpdatedProduct ON UpdatedProduct.ProductId = Product.ProductId

So what have I done there? Well the first two lines instruct the dbms to update the Product table, altering the price. The second two lines provide a join of the 2 tables we are using and an interim relation that the Update can use as it's source. A table of 250,000 records took 3 seconds to update with this method but over 30 minutes using a cursor so the benefits are huge.