Tutorial :Complex SQL query help on aggregating values for nested subquery



Question:

I have people, companies, employees, events and event kinds. I'm making a report/followup sheet where people, companies and employees are the rows, and the columns are event kinds.

Event kinds are simple values describing: "Promised Donation", "Received Donation", "Phoned", "Followed up" and such. Event kinds are ordered:

CREATE TABLE event_kinds (    id,    name,    position);  

Events hold the actual reference to the event:

CREATE TABLE events (    id,    person_id,    company_id,    referrer_id,    event_kind_id,    created_at);  

referrer_id is another reference to people. It is the person which sent the information/tip along, and is an optional field, although I sometimes want to filter on an event_kind that has a specific referrer, while I don't for other event kinds.

Notice I don't have an employee ID reference. The reference exists, but is implied. I have application code to validate that person_id and company_id really reference an employee record. The other tables are pretty basic:

CREATE TABLE people (    id, name);    CREATE TABLE companies (    id, name);    CREATE TABLE employees (    id, person_id, company_id);  

I'm trying to achieve the following report:

                           Referrer       Phoned     Promised   Donated      Francois                            Feb 16th   Feb 20th   Mar 1st      Apple (Steve Jobs)   Steve Ballmer                        Mar 3rd      IBM                  Bill Gates     Mar 7th  

The first row is a people record, the 2nd is an employee, and the 3rd is a company. If I asked for referrer Bill Gates for Phoned event kinds, I'd only see the 3rd row, while asking for Steve and Phoned would return no rows.

Right now, I do 3 queries, one for companies, one for people and a last one for employees. I want the event kind columns to be ordered, but I do that in application code and show it properly there. Here's where I'm at so far:

SELECT companies.id,         companies.name,         (SELECT events.id FROM events WHERE events.referrer_id = 1470 AND events.company_id = companies.id AND events.person_id IS NULL AND events.event_kind_id = 9 ORDER BY created_at DESC LIMIT 1) event_kind_9,         (SELECT events.id FROM events WHERE events.company_id = companies.id AND events.person_id IS NULL AND events.event_kind_id = 10 ORDER BY created_at DESC LIMIT 1) event_kind_10,         (SELECT events.created_at FROM events WHERE events.referrer_id = 1470 AND events.company_id = companies.id AND events.person_id IS NULL AND events.event_kind_id = 9 ORDER BY created_at DESC LIMIT 1) event_kind_9_order  FROM "companies"    SELECT people.id,         people.name,         (SELECT events.id FROM events WHERE events.referrer_id = 1470 AND events.company_id IS NULL AND events.person_id = people.id AND events.event_kind_id = 9 ORDER BY created_at DESC LIMIT 1) event_kind_9,         (SELECT events.id FROM events WHERE events.company_id IS NULL AND events.person_id = people.id AND events.event_kind_id = 10 ORDER BY created_at DESC LIMIT 1) event_kind_10,         (SELECT events.created_at FROM events WHERE events.referrer_id = 1470 AND events.company_id IS NULL AND events.person_id = people.id AND events.event_kind_id = 9 ORDER BY created_at DESC LIMIT 1) event_kind_9_order  FROM "people"    SELECT employees.id,         employees.company_id,         employees.person_id,         (SELECT events.id FROM events WHERE events.referrer_id = 1470 AND events.company_id = employees.company_id AND events.person_id = employees.person_id AND events.event_kind_id = 9 ORDER BY created_at DESC LIMIT 1) event_kind_9,         (SELECT events.id FROM events WHERE events.company_id = employees.company_id AND events.person_id = employees.person_id AND events.event_kind_id = 10 ORDER BY created_at DESC LIMIT 1) event_kind_10,         (SELECT events.created_at FROM events WHERE events.referrer_id = 1470 AND events.company_id = employees.company_id AND events.person_id = employees.person_id AND events.event_kind_id = 9 ORDER BY created_at DESC LIMIT 1) event_kind_9_order  FROM "employees"  

I rather suspect I'm doing this wrong. There should be an "easier" way to do it.

One other filter criteria would be to filter on people/company names: WHERE LOWER(companies.name) LIKE '%apple%'.

Note that I'm ordering by the dates of event_kind_9 here, and a secondary sort is by person/company name.

To summarize: I want to paginate the result set, find the latest event for each cell, order the result set by the date of the latest event, and by company/person name, filter by referrer in some event kinds, but not others.

For reference, I'm using PostgreSQL, from Ruby, ActiveRecord/Rails. The solution is pure SQL though.


Solution:1

Is it possible to have multiple rows for the same person/company and event_kind_id (e.g. Bill Gates Phoned on 07-Mar and 09-Mar)? If not, you can do something like:

Select Coalesce(People.name, Companies.name) As Name      , Referrers.name      , Min(Case When EventKinds.name = 'Phoned' Then Events.created_at End) As Phoned      , Min(Case When EventKinds.name = 'Promised' Then Events.created_at End) As Promised      , Min(Case When EventKinds.name = 'Donated' Then Events.created_at End) As Donated  From Events      Join EventKinds          On EventKinds.id = Events.event_kind_id      Left Join People As Referrers          On Referrers.id = Events.referrer_id      Left Join People          On People.id = Events.person_id      Left Join Companies          On Companies.id = Events.company_id  -- Where Companies.Name Like 'foo%'  Group By Coalesce(People.name, Companies.name), Referrers.name  

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