Generate rows where none exist - sql

Question

I'm a little stumped on how to generate rows when none exist for specified conditions. Apologies for the formatting since I don't know how to write tables in SO posts, but let's say I have data that looks like this:
TimePeriodID CityspanSiteKey Mean_Name Mean
2 123 Social Environment 4
2 123 Youth with Adults 3.666666746
2 123 Youth with Peers 3.5
4 123 Social Environment 2.75
4 123 Youth with Adults 2.555555582
4 123 Youth with Peers 3.5
There are a few other Mean_Name values which I would like to include in every single time period ID, but just a Mean value of NULL, like the following:
TimePeriodID CityspanSiteKey Mean_Name Mean
2 123 Social Environment 4
2 123 Youth with Adults 3.666666746
2 123 Youth with Peers 3.5
2 123 Staff Build Relationships and Support Individual Youth NULL
2 123 Staff Positively Guide Behavior NULL
4 123 Social Environment 2.75
4 123 Youth with Adults 2.555555582
4 123 Youth with Peers 3.5
4 123 Staff Build Relationships and Support Individual Youth NULL
4 123 Staff Positively Guide Behavior NULL
5 123 Social Environment 2.75
5 123 Youth with Adults 2.555555582
5 123 Youth with Peers 3.5
5 123 Staff Build Relationships and Support Individual Youth NULL
5 123 Staff Positively Guide Behavior NULL
6 123 Social Environment NULL
6 123 Youth with Adults NULL
6 123 Youth with Peers NULL
6 123 Staff Build Relationships and Support Individual Youth NULL
6 123 Staff Positively Guide Behavior NULL
What's the best way to go about doing this? I don't think CASEing will be of much use since these records don't exist.

Answer 1

You seem to want a cross join and then left join. Not all values are in your original data, so you might as well construct them:
select ti.timeperiod, c.CityspanSiteKey, m.mean_name, t.mean
from (values (2), (4), (5), (6)
) ti(timeperiod) cross join
(values (123)
) c(CityspanSiteKey) cross join
(values ('Social Environment'), ('Youth with Adults'), ('Youth with Peers'), ('Staff Build Relationships and Support Individual Youth'), ('Staff Positively Guide Behavior')
) m(mean_name) left join
t
on t.timeperiod = ti.timeperiod and
t.CityspanSiteKey = c.CityspanSiteKey and
t.mean_name = m.mean_name;
You can use subqueries or existing tables instead of the values() clause.