sql - not able to retrieve if the table column has null value -

i have stored procedure find customer, working fine. if customer_city_name null in table not able retrieve row. sp fails . how have result if customer_city_name or customer_country_code is null

exec findcustomer null,'%',null,null, 

sp code:

create procedure  findcustomer   @customernumber nvarchar(100),   @customernamepattern nvarchar(35),   @customercitynamepattern nvarchar(35),   @customercountrycode nvarchar(5)  begin declare @sql nvarchar(4000) set @sql = ' select c.customer_number, c.customer_name, c.postal_address_identifier, c.customer_street_or_road_name, c.customer_city_name, c.customer_territory_code, c.customer_postal_code, c.customer_country_code, c.telephone_number, c.mobile_telephone_number, c.fax_number, c.email_address  customer c   c.customer_number ' +         case when @customernumber not null              '''' + @customernumber + ''''              else 'c.customer_number'         end + ' , c.customer_name ' +         case when @customernamepattern not null             '''' + @customernamepattern + ''''             else 'c.customer_name'         end + ' , c.customer_city_name ' +         case when @customercitynamepattern not null             '''' +@customercitynamepattern + ''''             else 'c.customer_city_name'         end + ' , c.customer_country_code ' +         case when @customercountrycode not null             '''' +@customercountrycode + ''''             else 'c.customer_country_code'         end  exec sp_executesql @sql 

@user2218371 should reconsider alternatives instead of dynamic sql. anyway, alternative code.

create procedure  findcustomer   @customernumber nvarchar(100),   @customernamepattern nvarchar(35),   @customercitynamepattern nvarchar(35),   @customercountrycode nvarchar(5)  begin declare @sql nvarchar(4000) set @sql = ' select c.customer_number, c.customer_name, c.postal_address_identifier, c.customer_street_or_road_name, c.customer_city_name, c.customer_territory_code, c.customer_postal_code, c.customer_country_code, c.telephone_number, c.mobile_telephone_number, c.fax_number, c.email_address  customer c   ' +      case when  @customernumber not null          ' c.customer_number ''' + @customernumber + '''' +              case when  @customernamepattern not null ' , ' else '' end         else ''      end  +     case when   @customernamepattern not null          ' c.customer_name ''' +  @customernamepattern + '''' +              case when  @customercitynamepattern not null ' , ' else '' end         else ''      end  +      case when   @customercitynamepattern not null          ' c.customer_city_name ''' +  @customercitynamepattern + '''' +              case when  +@customercountrycode not null ' , ' else '' end         else ''      end  +      case when    @customercountrycode not null          ' c.customer_country_code ''' +  @customercountrycode + ''''         else ''      end       exec sp_executesql @sql