Hello Umesh,
Check here, for more information: https://www.c-sharpcorner.com/article/different-ways-to-handle-null-in-sql-server/
Replaces NULL with the specified replacement value
Evaluates the arguments in order and returns the current value of the first expression that initially doesn’t evaluate to NULL. For example, SELECT COALESCE(NULL, NULL, ‘third_value’, ‘fourth_value’); returns the third value because the third value is the first value that isn’t null.
Replaces NULL with the specified replacement valueEvaluates the arguments in order and returns the current value of the first expression that initially doesn't evaluate to NULL. For example, SELECT COALESCE(NULL, NULL, 'third_value', 'fourth_value'); returns the third value because the third value is the first value that isn't null.
IsNull can work for if value is null then use the next value, coalesce is take the first value which is not null of all the provided values.
The basic difference between them is isnull is used for replacing null from some user-friendly value, whereas coalesce is used for return first non nullable value from the column or list.
Like if coalesce found empty string first so it will return the same