CASE statement in SQL, The case statement in SQL returns a value on a specified condition. We can use a Case statement in select queries along with Where, Order By and Group By clause, The CASE statement goes through conditions and returns a value when the first condition is IF-THEN-ELSE statement. So, once a condition is true, it will stop reading and return the result. If no conditions are true, it returns the value in the ELSE clause. A CASE statement can return only one value. CASE by definition only returns a single value.
CASE WHEN condition1 THEN result1 WHEN condition2 THEN result2 WHEN condition3 THEN result3 WHEN conditionN THEN resultN ELSE result END
SELECT CName, City, Country FROM Customer ORDER BY (CASE WHEN City IS NULL THEN Country ELSE City END)