The coalesce function provides one way to substitute constant values for null values.
Is a function that returns the first non-NULL value from a series of expressions. The expressions are evaluated in the order in which they are specified, and the result of the function is the first value that is not null. The result of the COALESCE function returns NULL only if all the arguments are null.
Data type determination of the resulting expression is different. ISNULL uses the data type of the first parameter, COALESCE follows the CASE expression rules and returns the data type of value with the highest precedence.
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