急用，求各位大侠帮助！解决方法_Sql Server

急用，求各位大侠帮助！！！
数据库有两个字段 A B ，分别代表两种状态；

现在要根实际情况，可以灵活的修改两个状态、、

假如A 有值，我把值赋给B ，假如B有值，我把值赋给A，两个都没值，我就默认赋给A

怎样写存储过程呢？？

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case when .. else .. end
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if ... else ....
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SQL code

goif OBJECT_ID('tbl')is not nulldrop table tblgocreate table tbl(A varchar(3),B varchar(3))goinsert tblselect 'aaa',null union allselect null,'bbb' union allselect null,null --declare @str varchar(3)set @str='bbb'update tblset 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 endselect * from tbl/*A    Baaa    bbbbbb    bbbbbb    NULL*/
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SQL codegoif OBJECT_ID('tbl')is not nulldrop table tblgocreate table tbl(A varchar(3),B varchar(3))goinsert tblselect 'aaa',null union allselect null,'bbb' union allselect null,null --测试declare @str varchar(3)set @str='new'update tblset 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 endselect * from tbl/*A    B-------------aaa    newnew    bbbnew    NULL*/    这样看得明显点，注意对比两个字段的值