I want to update a field to the value of another field found in another row of the same table. I think this is possible, but i am not sure what the syntax would be. Each row has it’s unique key index.

I suppose my logic would be as follows:
UPDATE Categories SET isPage= (SELECT FROMCategories.isPage WHERE CatID=13) WHERE CatID=15

So its setting the value of isPage at row(CatID) 15, to whatever the value of isPage is at row(CatID) 13.

But, as I said before , the syntax is wrong however, and nearly an hour on google has proved fruitless. Any help would be greatly appreciated