go
if OBJECT_ID('tbl')is not null
drop table tbl
go
create table tbl(
A varchar(3),
B varchar(3)
)
go
insert tbl
select 'aaa',null union all
select null,'bbb' union all
select null,null 
--
declare @str varchar(3)
set @str='bbb'
update tbl
set A=case when A is null then @str
      when A is not null and B is null then A else A end,
    B=case when A is null then B
      when A is not null and B is null then @STR else B end
select * from tbl
/*
A    B
aaa    bbb
bbb    bbb
bbb    NULL
*/

------解决方案--------------------

SQL code

go
if OBJECT_ID('tbl')is not null
drop table tbl
go
create table tbl(
A varchar(3),
B varchar(3)
)
go
insert tbl
select 'aaa',null union all
select null,'bbb' union all
select null,null 
--测试
declare @str varchar(3)
set @str='new'
update tbl
set A=case when A is null then @str
      when A is not null and B is null then A else A end,
    B=case when A is null then B
      when A is not null and B is null then @STR else B end
select * from tbl
/*
A    B
-------------
aaa    new
new    bbb
new    NULL
*/
    

这样看得明显点，注意对比两个字段的值