Question

I have a configuration table, that holds the values selected by a specific user. If no values where entered, then the default should be returned. But there’s a catch: the user belong to a speciffic group, and each group has it’s own defaul.

What I need is a single query that returns the user’s values, or the default for his/her group if no data found.

The config tables structure is quite simple:

CONFIGURATION: ConfigurationId (PK), GroupId (FK), UserId (FK), ConfigurationDetailId (FK);
CONFIGURATIONDETAIL: ConfigurationDetailId (PK), ConfigValue1, ConfigValue2

A user may belong to more than one group, and have different configurations for each. Those configs are considered different (have their own ConfigurationDetailId) even if their values are the same.
When I look for the data, I always have the GroupId and the UserId

This works, but I wanted something more “elegant”… What I don’t like is the TOP 1 and ORDER BY I need to eliminate the default row when a user specific row exists:

SELECT TOP 1 
  FROM Config C
 WHERE C.GroupId = @GroupId
   AND (C.UserId = @UserId OR C.UserId IS NULL)
 ORDER BY C.UserId DESC

Any thoughts?

Answer 1

Perhaps try a self join, depending on your table structure.

create table config
 ( GroupID int, 
   UserID int, 
   ConfigId int, 
   ConfigValue varchar(50),
   IsDefault int );

insert into config (groupid, userid, configid, configvalue, isdefault)
values (1, NULL, 1, 'A', 1);
insert into config (groupid, userid, configid, configvalue, isdefault)
values (1, NULL, 2, 'B', 1);
insert into config (groupid, userid, configid, configvalue, isdefault)
values (1, NULL, 3, 'C', 1);
insert into config (groupid, userid, configid, configvalue, isdefault)
values (1, 1, 1, 'A1', 0);
insert into config (groupid, userid, configid, configvalue, isdefault)
values (1, 1, 2, 'B1', 0);
insert into config (groupid, userid, configid, configvalue, isdefault)
values (1, 1, 3, 'C1', 0);
insert into config (groupid, userid, configid, configvalue, isdefault)
values (1, 2, 1, 'A2', 0);

select 
   COALESCE(c1.configid, c2.configid) as configid,
   COALESCE(c1.configvalue, c2.configvalue) as configvalue
from config c1 right join config c2
 on (c1.groupid = c2.groupid 
     and c1.configid = c2.configid
     and c1.isdefault = 0 and c1.userid = 2)
where c2.isdefault = 1 and c2.groupid = 1;

select 
   COALESCE(c1.configid, c2.configid) as configid,
   COALESCE(c1.configvalue, c2.configvalue) as configvalue
from config c1 right join config c2
 on (c1.groupid = c2.groupid 
     and c1.configid = c2.configid
     and c1.isdefault = 0 and c1.userid = 1)
where c2.isdefault = 1 and c2.groupid = 1;

Results for userid = 2

CONFIGID    CONFIGVALUE
1           A2
2           B
3           C

Results for userid = 1

CONFIGID    CONFIGVALUE
1           A1
2           B1
3           C1

I’ve built a SQLFiddle for this so you can see it for yourself.