Foreign key on part of composite PK - sql

Question

I have a problem with finding how to declare the following type of association :
Say I have a table "Weekly" as such :
Weekly {
Id : Int <= PK
Week : Int
Year : Int
}
And a table "Monthly" :
Monthly{
Id : Int <= PK
Month: Int
Year : Int
}
I also have a "WeekMonth" Table :
Monthly{
Week : Int <= PK
Month : Int <= PK
Year : Int <= PK
}
As you my have guessed, i whant to be able to link the Weekly with WeekMonth and Monthly with WeekMonth too.
However, i can't seam to be able to do this : a foreign key on part of the composite primary key. Nevertheless, in my WeekMonth table, both the year and week and the year and month field are obviouly unique, so it should be able to work.
I've tried multiple approch to this problem , but as the custom mapping of week per month is a business need, I a bit stuck with it

Answer 1

in my WeekMonth table, both the year and week and the year and month field are obviouly unique
That isn't true. 'Year and week' may be unique, but it depends what 'week' is here - if it's the week within the month (i.e. 1-5) then it is not unique. If it's the week within the year (1-53) then it is; but you don't have a unique or primary key on that combination. And 'year and month' is not unique, as you will have multiple entries - either 4 or 5 - for each combination.
If you have a composite primary (or unique) key then a foreign key has to refer to all of the columns in that PK - otherwise they would not necessarily be unique.
A natural key isn't really working for you here. As well as not allowing the relationships you want, you're duplicating data in the parent and child tables. It would be better to have a synthetic key, e.g. set from a sequence:
WeekMonth{
WeekMonth_Id : Int <= PK (synthetic, e.g. from sequence)
Week : Int <= }
Month : Int <= } UK
Year : Int <= }
}
Weekly {
Weekly_Id : Int <= PK
WeekMonth_Id : Int <= FK to WeekMonth
}
Monthly{
Monthly_Id : Int <= PK
WeekMonth_Id : Int <= FK to WeekMonth
}
You don't need to duplicate the year/month/week values in the child tables as you can get them from the parent. And you shouldn't duplicate them, as you can't easily guarantee that the match the related parent record, as well as for general normalisation reasons.
I'm assuming you have other data in the weekly and monthly tables, otherwise they would be a bit pointless; any other table that has an FK to one of those could use an FK to WeekMonth instead.
If you do want to have the individual year/month/week values duplicated in the child tables then you will need separate unique keys for those combinations, in addition to your current PK. So you'd modify WeekMonth to have a unique key on year and month (which may be possible, depending what 'week' represents), and another unique key on year and month - but as that is not a unique combination you can't create that key.

Answer 2

Assuming that the WeekMonth table has Week values 1 through 53 for the year then:
CREATE TABLE WeekMonth(
Week INT,
Month INT,
Year INT,
CONSTRAINT WeekMonth__W_M_Y__PK PRIMARY KEY ( Week, Month, Year ),
CONSTRAINT WeekMonth__W_Y__PK UNIQUE ( Week, Year )
);
CREATE TABLE Monthly(
ID INT PRIMARY KEY,
Month INT,
Year INT,
FirstWeek INT GENERATED ALWAYS
AS ( TO_NUMBER(
TO_CHAR(
NEXT_DAY(
TO_DATE( month||'-'||year, 'MM-YYYY' ) - 1,
'MONDAY'
),
'WW'
)
)
),
CONSTRAINT Monthly__M_Y__PK FOREIGN KEY ( FirstWeek, Month, Year )
REFERENCES WeekMonth( Week, Month, Year )
);
CREATE TABLE Weekly(
ID INT PRIMARY KEY,
Week INT,
Year INT,
CONSTRAINT Weekly__W_Y__PK FOREIGN KEY ( Week, Year )
REFERENCES WeekMonth( Week, Year )
);