How does MySQL CASE work?

Question

I know that SQL\'s CASE syntax is as follows:

CASE
    WHEN search_condition THEN statement_list
    [WHEN search_condition THEN statement_list]         


        

Answer 1

CASE in MySQL is both a statement and an expression, where each usage is slightly different.

As a statement, CASE works much like a switch statement and is useful in stored procedures, as shown in this example from the documentation (linked above):

DELIMITER |

CREATE PROCEDURE p()
  BEGIN
    DECLARE v INT DEFAULT 1;

    CASE v
      WHEN 2 THEN SELECT v;
      WHEN 3 THEN SELECT 0;
      ELSE
        BEGIN -- Do other stuff
        END;
    END CASE;
  END;
  |

However, as an expression it can be used in clauses:

SELECT *
  FROM employees
  ORDER BY
    CASE title
      WHEN "President" THEN 1
      WHEN "Manager" THEN 2
      ELSE 3
    END, surname

Additionally, both as a statement and as an expression, the first argument can be omitted and each WHEN must take a condition.

SELECT *
  FROM employees
  ORDER BY
    CASE 
      WHEN title = "President" THEN 1
      WHEN title = "Manager" THEN 2
      ELSE 3
    END, surname

I provided this answer because the other answer fails to mention that CASE can function both as a statement and as an expression. The major difference between them is that the statement form ends with END CASE and the expression form ends with just END.