update test    
set 字段1=case 
when 条件1 then 值1    
when 条件2 then 值2    
end  

UPDATE group_user SET unread = CASE 
WHEN is_enter =1 THEN 0 
WHEN is_enter =0 THEN unread +1 END 
WHERE group_id =6

SELECT
CASE
WHEN price IS NULL THEN 'Not yet priced'
WHEN price < 10 THEN 'Very Reasonable Title'
WHEN price >= 10 and price < 20 THEN 'Coffee Table Title'
ELSE 'Expensive book!'
END AS "Price Category",
CONVERT(varchar(20), title) AS "Shortened Title"
FROM pubs.dbo.titles
ORDER BY price

SELECT au_fname, au_lname,
   CASE state
      WHEN 'CA' THEN 'California'
      WHEN 'KS' THEN 'Kansas'
      WHEN 'TN' THEN 'Tennessee'
      WHEN 'OR' THEN 'Oregon'
      WHEN 'MI' THEN 'Michigan'
      WHEN 'IN' THEN 'Indiana'
      WHEN 'MD' THEN 'Maryland'
      WHEN 'UT' THEN 'Utah'
        END AS StateName
FROM pubs.dbo.authors
ORDER BY au_lname