SQL Server 查询语句怎样定义多种不同类型的过滤条件

Drop Table Emp;Create Table Emp(Id int not null primary key,Name varchar(40) not null,City varchar(40))Insert Into Emp Values(1,'Chris','Xi''an'),(2,'Sonia','Nanyang'),(3,'Alvin','Linfen'),(4,'Sherman','Baoji'),(5,'Joanna','Xianyang'),(6,'Morris','Xi''an'),(7,'Liz','Xianyang'),(8,'Kenny',null);Declare @City varchar(40);--Set @City='Nanyang';Set @City='All';Select * From Emp Where City = @Cityor (@City = 'All' and City is not null)Declare @City varchar(40);--Set @City='Nanyang';Set @City='''Nanyang'',''Xi''''an''';exec('Select * From Emp Where City in ('+@City+')');