The page explains the SQL CASE expression, which is used to add if-then-else logic within SQL queries.

• The CASE expression checks conditions one by one and returns a value for the first condition that is true, similar to an if-then-else statement in other programming languages.
• If none of the conditions is true, the value after ELSE is returned. If there is no ELSE and no condition is true, it returns NULL.
• Syntax:

  CASE
    WHEN condition1 THEN result1
    WHEN condition2 THEN result2
    ...
    ELSE result
  END;
  

• Examples include:
  • Returning a label based on the value of a column (e.g., if Quantity > 30, return 'The quantity is greater than 30').
  • Using CASE in an ORDER BY clause to sort by one column unless it's NULL, in which case it sorts by another column.

The page also includes interactive examples and a quiz question to reinforce understanding.[1]

1

SQL CASE Expression

The SQL CASE Expression

The CASE expression goes through conditions and returns a value when the first condition is met (like an 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.

If there is no ELSE part and no conditions are true, it returns NULL.

CASE Syntax

CASE
    WHEN condition1 THEN result1
    WHEN condition2 THEN result2
    WHEN conditionN THEN resultN
    ELSE result
END;

Demo Database

Below is a selection from the "OrderDetails" table in the Northwind sample database:

SQL CASE Examples

The following SQL goes through conditions and returns a value when the first condition is met:

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 AS QuantityText
FROM OrderDetails;

The following SQL will order the customers by City. However, if City is NULL, then order by Country:

SELECT CustomerName, City, Country  
FROM Customers  
ORDER BY  
(CASE  
    WHEN City IS NULL THEN Country  
    ELSE City  
END);