Question

I have 3 tabels with the following definitions

people
------
- wid 
- name

types
-----
- guid
- type

mapping
-------
- guid
- wid

The people table has the list of people

The types table the type information for each row present in the people table. If a person belongs to more than one type, then two rows are present in the types table.

The mapping table provides a mapping between people and types table.

Now to find out who are the people of type ‘politician’ I can use the following query.

select name from people inner join 
(mapping inner join types on mapping.guid = types.guid) 
on people.wpid = mapping.wpid where types.type = 'politician'

But now I want to find out what are the other types a politician belongs. I know that I have to use group by and having clause. But I am not able to come up with the query. How to write this query?

Answer 1

Try:

select p.name, t2.type
from types t1
join mapping m1 on m1.guid = t1.guid
join people p on p.wpid = m1.wpid 
join mapping m2 on p.wpid = m2.wpid 
join types t2 on m2.guid = t2.guid
where t1.type = 'politician'
order by 1, 2

– for a list of all politicans and all the types that they belong to.

Alternatively, if you just want a list of all politicians and the number of different types that they belong to, try:

select p.name, count(*)
from mapping m1
join people p on p.wpid = m1.wpid 
join mapping m2 on p.wpid = m2.wpid 
where m1.guid = 1 /* replace 1 with appropriate guid for politicians */
group by p.name
order by 1