sql server 2005 - stored procedure with a default value -

i have 2 tables: 1 called city (city_id,name, latitude, longitude) , other measures (city_id, measure_date, temperature, humidity, windspeed, airpressure).

city_id primary key in city , foreign key in measures. city_id , measure_date primary key too.

i trying write stored procedure in sql server 2005 allows me calculate averages of 4 measurements have in database specific city. , if city has no records in measures, want default value null.

how should make set default value null cities? thank :)

here code did:

set ansi_nulls on set quoted_identifier on go  alter procedure [dbo].[low_temp]      @temp real begin     select          avg(m.temperature), avg(m.humidity),          avg(m.windspeed), avg(m.airpressure)              city c, measures m               m.city_id = @temp end 

just use left join:

set ansi_nulls on set quoted_identifier on go  alter procedure [dbo].[low_temp]      @temp real begin     select avg(m.temperature),             avg(m.humidity),             avg(m.windspeed),             avg(m.airpressure)     city c     left join measures m on(c.city_id = m.city_id)              c.city_id = @temp end