SQL Server one-to-many relational IF query - sql

Question

I have an employees table that has many employee_records. The employee_records table has a column named event_type and it is an enum that can be either hire-date, promotion, termination, title-change, or rehire. I am attempting to calculate an employees total time employed and make some calculations based on how long they have been employed.
How can I add a column that gives me the total days that they have been employed?
Essentially, I need to see if they have a record with the event_type = termination and if they do, then I need to see if they have a rehire date, and if they do, then I need to use their rehire date as the first day of their employment and calculate their time of employment that way.
As for a result, I simply need a days_employed column that reflects the actual amount of days they have been employed.
Here is what I have so far.
SELECT
employees.id,
first_name,
last_name,
email,
event_type,
CASE
WHEN DATEDIFF(DAY, employee_records.created_at, SYSDATETIME()) < 365 * 5
THEN 1
WHEN DATEDIFF(DAY, employee_records.created_at, SYSDATETIME()) < 365 * 10
THEN 2
ELSE 3
END AS benfits_type,
DATEDIFF(DAY, employee_records.created_at, SYSDATETIME()) AS days_employed,
employee_records.created_at AS hire_date
FROM
employees
JOIN
employee_records ON employees.id = employee_records.employee_id
ORDER BY
employees.id ASC;

Answer 1

Here is an example of how you could do this. I'll post the query first and then walk through my explanation. If I understood you correctly, you were not looking for total days the employee was hired, but rather the total days of the employee's most recent employment at the company (Max hire date to max termination date or today).
;WITH hired
AS (SELECT employee_records.employee_id id,
Max(employee_records.created_at) created_at
FROM employee_records
WHERE event_type = 'hire-date'
GROUP BY employee_records.employee_id),
latest
AS (SELECT employees.id
id,
Isnull(Cast(Max(employee_records.created_at) AS DATE), Getdate()
)
created_at
FROM employees
LEFT JOIN employee_records
ON employees.id = employee_records.employee_id
AND employee_records.event_type = 'termination'
GROUP BY employees.id)
SELECT *,
Datediff(day, hired, latestday) DaysEmployeed
FROM (SELECT hired.id,
hired.created_at AS Hired,
CASE
WHEN hired.created_at > latest.created_at THEN Cast(
Getdate() AS DATE)
ELSE latest.created_at
END AS LatestDay
FROM hired
INNER JOIN latest
ON hired.id = latest.id) JoinedCTEs
First of all I know you mentioned event_type is an integer, but for easy explanation I used a varchar.
Two CTEs to start.
First there is "hired" which will get you the latest hire date. So if an employee has multiple hire dates, it grabs the latest date.
Second there is "latest" which is the latest date an employee has a termination date, but also uses today's date as a placeholder date if an employee has never been terminated.
The final query joins the two CTEs and does a datediff by day to determine how many days an employee has been at the company. If the termination date is earlier than the hire date (An employee who was hired, terminated, rehired and is still with the company), it will take today's date as the latest date to count.