Question: How do I JOIN multiple (3+) tables, which all have SCD type 2 validFrom/validTo dates in them?
I have the following tables:
-- table 1
CREATE TABLE dbo.Clients (
clientCode varchar(10) NOT NULL,
startDate date NOT NULL,
[name] varchar(200) NOT NULL,
CONSTRAINT PK_Clients PRIMARY KEY CLUSTERED (clientCode, startDate)
);
-- table 2
CREATE TABLE dbo.Projects (
clientCode varchar(10) NOT NULL, --- Each project belongs to a client.
projectCode varchar(10) NOT NULL,
startDate date NOT NULL,
[name] varchar(200) NOT NULL,
CONSTRAINT PK_Projects PRIMARY KEY CLUSTERED (projectCode, startDate)
);
.. with the following dummy data:
-- dummy data
INSERT INTO dbo.Clients (clientCode, startDate, [name])
VALUES ('A', {d '2010-01-01'}, 'Client A (first)'),
('A', {d '2011-04-01'}, 'Client A (second)'),
('A', {d '2011-09-01'}, 'Client A (third)'),
('A', {d '2012-02-01'}, 'Client A (fourth)'),
('A', {d '2014-01-01'}, 'Client A (fifth)'),
('B', {d '2010-01-01'}, 'Client B (first)'),
('B', {d '2011-02-01'}, 'Client B (second)'),
('B', {d '2011-08-01'}, 'Client B (third)'),
('B', {d '2011-12-01'}, 'Client B (fourth)'),
('B', {d '2012-11-01'}, 'Client B (fifth)');
-- dummy data
INSERT INTO dbo.Projects (clientCode, projectCode, startDate, [name])
VALUES ('A', '1', {d '2010-01-15'}, 'Project 1, first revision'),
('A', '1', {d '2012-04-22'}, 'Project 1, second revision'),
('A', '2', {d '2010-02-08'}, 'Project 2, first revision'),
('A', '2', {d '2010-09-12'}, 'Project 2, second revision'),
('A', '2', {d '2012-08-18'}, 'Project 2, third revision'),
('B', '3', {d '2011-04-01'}, 'Project 3, first revision'),
('B', '3', {d '2011-12-01'}, 'Project 3, second revision'),
('B', '3', {d '2014-02-28'}, 'Project 3, third revision');
Using these two tables, we generate startDate and endDate intervals:
--- Clients:
WITH c (clientCode, [name], startDate, endDate) AS (
SELECT clientCode, [name], startDate,
--- Find the next record's startDate, ordered by startDate.
LEAD(startDate, 1, {d '2099-12-31'}) OVER (
PARTITION BY clientCode
ORDER BY startDate) AS endDate
FROM dbo.Clients),
--- Projects:
p (projectCode, clientCode, [name], startDate, endDate) AS (
SELECT projectCode, clientCode, [name], startDate,
--- Find the next record's startDate, order by startDate
LEAD(startDate, 1, {d '2099-12-31'}) OVER (
PARTITION BY projectCode
ORDER BY startDate) AS endDate
FROM dbo.Projects)
SELECT c.clientCode, c.[name] AS clientName,
p.projectCode, p.[name] AS projectName,
--- Start date is the last of (c.startDate, p.startDate)
(CASE WHEN c.startDate<p.startDate THEN p.startDate ELSE c.startDate END) AS startDate,
--- End date is the first of (c.endDate, p.endDate)
(CASE WHEN c.endDate<p.endDate THEN c.endDate ELSE p.endDate END) AS endDate
FROM c
LEFT JOIN p ON
c.clientCode=p.clientCode AND
c.startDate<p.endDate AND
c.endDate>p.startDate
-- IF two new tables were introducted (t3 and t4), would the following JOINS work?
-- LEFT JOIN dbo.Table3 as t3
-- on p.clientCode = t3.clientcode AND
-- p.startdate<t3.endate AND
-- p.endDate>t3.startdate
-- LEFT JOIN dbo.Table4 as t4
-- on t3.toolId = t4.toolid AND --> toolId is a new key that I need for the join, since t4 does not have clientCode
-- t3.startdate<t4.enddate AND
-- t3.enddate>t4.startdate
ORDER BY c.clientCode, p.projectCode, 5;
My problem: In the bottom of the above query, I commented out the LEFT JOINS, which I will have to make when more SCD2 tables are introduced. I am unsure if the commented out LEFT JOINS i made will work. Do you see any issues with it?
Adding more JOINS maybe conflicts the CASE WHEN statement used in the above query..:
--- Start date is the last of (c.startDate, p.startDate)
(CASE WHEN c.startDate<p.startDate THEN p.startDate ELSE c.startDate END) AS startDate,
--- End date is the first of (c.endDate, p.endDate)
(CASE WHEN c.endDate<p.endDate THEN c.endDate ELSE p.endDate END) AS endDate
This CASE when statement is used because I want no two intervals to reference the same date. So, the output interval is defined by the larger of (a.startTime, b.startTime) and the smaller of (a.endTime, b.endTime).
I see an issue here, since this CASE WHEN statement only evaluates startDate and endDate intervals from 2 tables and not 3, 4 or more tables.
How can this perhaps be solved?
