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select id, ifnull(marks,0) from students;
#if marks=NULL, replaces value with 0
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# Id ProductName UnitPrice UnitsInStock UnitsOnOrder
# 1 Jarlsberg 10.45 16 15
# 2 Mascarpone 32.56 23
SELECT ProductName, UnitPrice * (UnitsInStock + IFNULL(UnitsOnOrder, 0)) FROM Products;