用 CASE.....WHEN 结构 case when 1=1 then 2 else 3 end
case when 更强大--超过两项 select case x when 'A' then 1 'B' then 2 'C' then 3 'D' then 4 'E' then 5 else 6 end as x from tablename--可以嵌套 select case x when 'A' then case when y<0 then 0 when y>=0 and y<=10 then y else 10 end else case when y<0 then -1 when y>=0 and y<=10 then 0 else 1 end end as xy from tablename等等
case when 1=1 then 2 else 3 end
select
case x when
'A' then 1
'B' then 2
'C' then 3
'D' then 4
'E' then 5
else 6
end as x
from tablename--可以嵌套
select
case x when
'A' then case when y<0 then 0
when y>=0 and y<=10 then y
else 10
end
else case when y<0 then -1
when y>=0 and y<=10 then 0
else 1
end
end as xy
from tablename等等