text: # Concept
  ## Swap
  
  # Solution 1 (self, wrong)
  UPDATE salary O
  SET O.sex = N.sex
  FROM salary N
  WHERE O.sex != N.sex;
  
  # Solution 2 (reference, correct in MySQL)
  update salary
  set sex = if (sex='m', 'f', 'm')
  
  # Solution 3 (self, correct in PostgreSQL)
  UPDATE Test 
  set sex = (CASE WHEN sex = 'm' THEN 'f' ELSE 'm' end);
  