declare @a int
declare @b int
set @a=-3
select @b=case when @a<0 then 0 else @a end
select @b as value

/*
value
0
*/

--你这样不行吗？？

------解决方案--------------------
SQL code

if object_id('[lsxf1988_Table]') is not null drop table [lsxf1988_Table]
create table [lsxf1988_Table]([AVAILABLE_STOCK] int)
insert [lsxf1988_Table]
select -1 union all
select 9 union all
select 8

declare @AVAILABLE_STOCK int
set  @AVAILABLE_STOCK=( select top 1 
case when AVAILABLE_STOCK<0 then 0
else AVAILABLE_STOCK end from [lsxf1988_Table])

select @AVAILABLE_STOCK as [库存]
/*
库存
-----------
0
*/