Left join not pulling through nulls where expected - sql

Question

I have a query that looks something like this (I've changed the table names):
select #user_id
, isnull(ur.rule_value, isnull(manr.rule_value, def.rule_value)) [rule_value]
, isnull(urt.name, isnull(manrt.name, def.name)) [rule_type]
from (select #user_id [user_id]
, rule.rule_value
, rule_type.name
from rule
join rule_type on rule_type.rule_type_id = rule.rule_type_id
where rule.user_id = 1) def
join user on user.user_id = def.user_id
join manager man on man.manager_id = user.manager_id
left join rule ur on ur.user_id = user.user_id
left join rule_type urt on urt.rule_type_id = ur.rule_type_id
left join rule manr on manr.manager_id = man.manager_id
left join rule_type manrt on manrt.rule_type_id = manr.rule_type_id
What I'm expecting is that when there isn't a rule for the user or the user's manager, then the default rule should be used. However, I'm only getting a result if the user has a rule.
I've tried left joining everything but to no avail, and the select statement for the def table brings back all the default rules.
What am I doing wrong?
#user_id is a variable.
Update
Example of schema
rule
rule_id user_id manager_id rule_value
1 1 1 27
2 1 1 24
3 1 1 25
4 1 1 44
5 1 1 88
1 2 4 2
2 2 4 23
3 2 4 18
3 NULL 4 19
4 NULL 4 20
5 NULL 4 21
rule_type
rule_id name
1 'Craziness'
2 'Number of legs'
3 'Hair ranking'
4 'Banana preference'
5 'Rule 5'
user
user_id manager_id ... other columns
1 1
2 4
3 4
manager
manager_id ... other columns
1
2
3
4
5
6
So if #user_id is 2 then I would expect the output
2, 2, 'Craziness'
2, 23, 'Number of legs'
2, 18, 'Hair ranking'
2, 20, 'Banana preference'
2, 21, 'Rule 5'
But if #user_id is 3 then I would expect the output
3, 27, 'Craziness'
3, 24, 'Number of legs'
3, 19, 'Hair ranking'
3, 20, 'Banana preference'
3, 21, 'Rule 5'

Answer 1

A regular join is actually a left inner join. What you actually want is an outer join, which fetches the result, even if it cannot join it with the other table.

Answer 2

To answer the question in the title, a LEFT JOIN does NOT act like a REGULAR JOIN. The LEFT JOIN is the same as the LEFT OUTER JOIN, meaning will also return the records in the LEFT table that don't match the ON criteria.

Answer 3

Get rid of this :
left join rule ur on ur.user_id = user.user_id
left join rule_type urt on urt.rule_type_id = ur.rule_type_id
left join rule manr on manr.manager_id = man.manager_id
left join rule_type manrt on manrt.rule_type_id = manr.rule_type_id
and replace it with this :
left join rule ur on ur.user_id = user.user_id or ur.user_id = man.manager_id
left join rule_type urt on urt.rule_type_id = ur.rule_type_id
then change your opening SELECT to :
select #user_id
, isnull(ur.rule_value, def.rule_value) [rule_value]
, isnull(urt.name, def.name) [rule_type]
Essentially what you're doing wrong is that you have one entity (user-manager-default) and you're trying to link it to a different entity (rule) via two different joins. If one join doesn't work but the other does, there's no matching NULL returned for your ISNULL to find. It's a little counter-intuitive.

Answer 4

I've split the query into two parts.
I think because the table structures are more complicated than I put into the original question, this means that CodeByMoonlight's answer isn't quite enough - it was giving me duplicate rows that I couldn't account for.
My solution is as follows (with respect to the details I gave in the question):
create table #user_rules
(
user_id int,
rule_value int,
rule_type varchar(255),
rule_type_id,
)
--Insert default values
insert into #user_rules
select #user_id [user_id]
, rule.rule_value
, rule_type.name
, rule_type.rule_type_id
from rule
join rule_type on rule_type.rule_type_id = rule.rule_type_id
where rule.user_id = 1
--Update table with any available values
update #user_rules
set rule_value = ur.rule_value
from user u
join manager m on u.manager_id = m.manager_id
join rule ur on ur.user_id = u.user_id or (ur.manager_id = man.manager_id and ur.user_id is null)
join rule_type urt on urt.rule_type_id = ur.rule_type_id
where urt.rule_type_id = #urse_rules.rule_type_id
and u.user_id = #user_id