SQL CASE Keyword
CASE
The CASE command is used is to 
create different output based on conditions.
The following SQL goes through several conditions and returns a value when the specified condition is met:
Example
  SELECT OrderID, Quantity,
CASE
    WHEN Quantity > 30 
  THEN 'The quantity is greater than 30'
    WHEN Quantity = 
  30 THEN 'The quantity is 30'
    ELSE 'The quantity is under 30'
END
FROM OrderDetails;
Try it Yourself »
The following SQL will order the customers by City. However, if City is NULL, then order by Country:
Example
  SELECT CustomerName, City, Country
FROM Customers
ORDER BY
(CASE
    
  WHEN City IS NULL THEN Country
    ELSE City
END);
Try it Yourself »
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