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 = @City
	or (@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+')');