update case when

UPDATE table
   SET 某字段=
          (CASE
              WHEN 条件1 AND 条件2
                 THEN '赋值1' 
              WHEN 条件1 AND 条件2 AND 条件3 
                 THEN '赋值2'
              WHEN 条件1
                 THEN '赋值3' 
           END
          )
WHERE 条件;

如果没有where条件那么“某字段”在CASE WHEN条件以外的都将被赋值为NULL