SQL adding multiple clauses on the JOIN - sql

Question

I've got the query below which almost works. It's returning the correct number of records and the way I want the records formatted. The thing is I need to return data based on a family so I want the returned data to include the father and mother on a single row of data if they belong to that family or two rows if they have a different familyID.
I've got the following query, the only issue I have is it is returning father and mother on the same line even if they are separated. I've looked at the code and I know why its happening, due to the way I have joined the father and mother to relations table. To handle separations I have a table called family and my relation table contains the familyID key. I need to somehow incorporate to check whether mothers familyID = father familyID somewhere where I make my join on the relation table for both parents, but I'm a little stuck.
I started this query with INNER JOINS, but changed to LEFT JOINS as some of the pupil data didn't have a both parents, so the data returned incorrect number of rows - however seems to work OK with LEFT JOINS.
SELECT p.pk_PupilID, p.Title, p.Forename, p.Surname,
father.pk_PersonID, father.Title, father.Forename, father.Surname,
mother.pk_PersonID, mother.Title, mother.Forename, mother.Surname,
ff.Salutation
FROM Pupil AS p
LEFT JOIN RELATION AS father_r
ON p.pk_PupilID = father_r.fk_PersonID
AND father_r.Relationship IN ('Father','Father')
LEFT JOIN PERSON AS father
ON father_r.fk_RelatedPersonID = father.pk_PersonID
LEFT JOIN RELATION AS mother_r
ON p.pk_PupilID = mother_r.fk_PersonID
AND mother_r.Relationship IN ('Mother','Stepmother')
LEFT JOIN PERSON AS mother
ON mother_r.fk_RelatedPersonID = mother.pk_PersonID
LEFT JOIN Family AS ff
father_r.fk_FamilyID = ff.pk_FamilyID
OR
mother_r.fk_FamilyID = ff.pk_FamilyID

Answer 1

This post is under the assumption the database is MySQL and I can use #variables within the query
I would start with a query of all possible "Parent" types of father, mother or other and sort their respective priority to a position type for all families and group by the person ID in the relationship. Since you appear to be pulling for ALL pupils, we can pre-gather all possible parents per person. Using SQLVariables, we can help pre-group possible entities regardless of parental type relation.
SELECT
Family.fk_PersonID,
Family.fk_FamilyID,
#nextParent := if( #lastPerson = Family.fk_PersonID
AND #lastFamily = Family.fk_FamilyID, #nextParent, 1 ) as parentSeq,
#lastPerson := Family.fk_PersonID as lastPersonSeq,
#lastFamily := Family.fk_FamilyID as lastFamilySeq,
if( #nextParent == 1, Family.pk_PersonID, " " ) as Parent1PersonID,
if( #nextParent == 1, Family.Relationship, " " ) as Parent1Type,
if( #nextParent == 1, Family.Title, " " ) as Parent1Title,
if( #nextParent == 1, Family.Forename, " " ) as Parent1Forename,
if( #nextParent == 1, Family.Surname, " " ) as Parent1Surname,
if( #nextParent == 2, Family.pk_PersonID, " " ) as Parent2PersonID,
if( #nextParent == 2, Family.Relationship, " " ) as Parent2Type,
if( #nextParent == 2, Family.Title, " " ) as Parent2Title,
if( #nextParent == 2, Family.Forename, " " ) as Parent2Forename,
if( #nextParent == 2, Family.Surname, " " ) as Parent2Surname
from
( select
#lastPerson := 0,
#lastFamily := 0,
#nextParent := 0 ) sqlvars,
( SELECT
fr.fk_PersonID,
fk_FamilyID,
fr.fk_RelatedPersonID,
fr.relationship,
whatParent.pk_PersonID,
whatParent.Title,
whatParent.Forename,
whatParent.Surname,
CASE when fr.relationship in ('Father', 'Stepfather' ) then 1
when fr.relationship in ('Mother', 'Stepmother' ) then 2 end as reltype
from
Relation fr
JOIN PERSON AS whatParent
ON fr.fk_RelatedPersonID = whatParent.pk_PersonID
where
fr.relationship in ('Father', 'Stepfather', 'Mother', 'Stepmother' )
order by
fr.fk_PersonID,
fr.fk_FamilyID,
CASE when fr.relationship in ('Father', 'Stepfather' ) then 1
when fr.relationship in ('Mother', 'Stepmother' ) then 2 end ) as Family
group by
Family.fk_PersonID,
Family.fk_FamilyID
The inner order by clause will force any Father / Stepfather in the first position and mother / step mother in second. This now qualifies for all possible conditions of things like f=father, sf=step, m=mother, sm=step (f/m, f/sm, sf/m, f/f, m/m, m/sm, etc). If you want other parental considerations, add them to the inner where clause and relation ship type sequence for priority positioning.
Due to the group by of per person, it SHOULD be able to ignore issues about not having aggregate columns in the group by clause... of which, you could always wrap each of the OTHER non aggreates to MAX() such as
MAX( #nextParent := if( #lastPerson = Family.fk_PersonID, #nextParent, 1 )) as parentSeq,
MAX( #lastPerson := Family.fk_PersonID ) as lastPersonSeq,
MAX( if( #nextParent == 1, Family.Relationship, " " )) as Parent1Type,
MAX( if( #nextParent == 1, Family.Title, " " )) as Parent1Title,
MAX( if( #nextParent == 1, Family.Forename, " " )) as Parent1Forename,
MAX( if( #nextParent == 1, Family.Surname, " " )) as Parent1Surname,
MAX( if( #nextParent == 2, Family.Relationship, " " )) as Parent2Type,
MAX( if( #nextParent == 2, Family.Title, " " )) as Parent2Title,
MAX( if( #nextParent == 2, Family.Forename, " " )) as Parent2Forename,
MAX( if( #nextParent == 2, Family.Surname, " " )) as Parent2Surname
So now, all the parental relationships are all rolled up to one row based on the fk_PersonID AND FAMILY. Now, just join that to the pupil's table, something like..
SELECT
p.pk_PupilID,
parents.fk_FamilyID,
p.Title,
p.Forename,
p.Surname,
Parents.Parent1PersonID,
Parents.Parent1Type,
Parents.Parent1Title,
Parents.Parent1Forename,
Parents.Parent1Surname,
Parents.Parent2PersonID,
Parents.Parent2Type,
Parents.Parent2Title,
Parents.Parent2Forename,
Parents.Parent2Surname,
ff.Salutation
FROM
Pupil AS p
LEFT JOIN ( entire query from above ) as Parents
ON p.pk_PupilID = Parents.fk_PersonID
LEFT JOIN Family AS ff
ON parents.fk_FamilyID = ff.pk_FamilyID
ORDER BY
p.pk_PupilID,
parents.fk_FamilyID