Coalesce returns the first non-null expression among its arguments.
Lets say we have to return a non-null from more than one column, then we can use COALESCE function.
SELECT COALESCE(hourly_wage, salary, commission) AS 'Total Salary' FROM wages
In this case,
If hourly_wage is not null and other two columns are null then hourly_wage will be returned.
If hourly_wage, commission are null and salary is not null then salary will be returned.
If commission is non-null and other two columns are null then commission will be returned.
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