php - Update a table sales with values from table products with WHERE clause in both Update and select query

Solution:

[...] how to update only wheresales.Amount= 0. I want columnAmount with values != 0 ignored .

Just add awhere clause:

UPDATE sales
SET Amount = (SELECT Selling_Price FROM products WHERE sales.Product = products.Productname)
WHERE Amount <> 0;
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Solution:

If you only want to update rows whereamount = 0, then you would addwhere clause to theupdate:

UPDATE sales
    SET Amount = (SELECT p.Selling_Price
                  FROM products p
                  WHERE sales.Product = p.Productname
                 )
    WHERE Amount = 0;
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IfAmount could beNULL and you want those rows updated as well, you would asOR Amount IS NULL.