Okay then, it's time to revise the most important aspects of CASE WHEN.

• A simple CASE WHEN has the following syntax:

  CASE column_name
    WHEN value1 THEN text1
    WHEN value2 THEN text2
    ...
    ELSE text_else
  END
  

• A searched CASE WHEN has the following syntax:

  CASE
    WHEN condition1 THEN text1
    WHEN condition2 THEN text2
    ...
    ELSE text_else
  END
  

• The ELSE part is optional.
• Remember to put the END clause at the end.
• You can use CASE WHEN with SUM to count multiple values in a single query:

  SUM(CASE WHEN x THEN 1 ELSE 0 END)

• Similarly, you can use CASE WHEN with COUNT to count multiple values in a single query:

  COUNT(CASE WHEN x THEN column END)