Considering values from one table as column header in another - sql

Question

I have a base table where I need to calculate the difference between two dates based on the type of the entry.
tblA
+----------+------------+---------------+--------------+
| TypeCode | Log_Date | Complete_Date | Pending_Date |
+----------+------------+---------------+--------------+
| 1 | 18/04/2016 | 19/04/2016 | |
| 2 | 10/04/2016 | 18/04/2016 | 15/04/2016 |
| 3 | 12/04/2016 | 19/04/2016 | |
| 4 | 15/04/2016 | 17/04/2016 | 16/04/2016 |
| 5 | 16/04/2016 | 21/04/2016 | |
| 1 | 19/04/2016 | 20/04/2016 | |
| 2 | 20/03/2016 | 31/03/2015 | |
| 3 | 25/03/2016 | 28/03/2016 | |
| 4 | 26/03/2016 | 27/03/2016 | |
| 5 | 27/03/2016 | 30/03/2016 | |
+----------+------------+---------------+--------------+
I have another look up table which has the column names to be considered based on the TypeCode.
tblB
+----------+----------+---------------+
| TypeCode | DateCol1 | DateCol2 |
+----------+----------+---------------+
| 1 | Log_Date | Complete_Date |
| 2 | Log_Date | Pending_Date |
| 3 | Log_Date | Complete_Date |
| 4 | Log_Date | Pending_Date |
| 5 | Log_Date | Complete_Date |
+----------+----------+---------------+
I am doing a simple DATEDIFF between two dates for my calculation. However I want to lookup which columns to consider for this calculation from tblB and apply it on tblA based on the TypeCode.
Resulting table:
For example: When the TypeCode is 2 or 4 then the calculation should be DATEDIFF(d, Log_Date, Pending_Date), otherwise DATEDIFF(d, Log_Date, Complete_Date)
+----------+------------+---------------+--------------+----------+
| TypeCode | Log_Date | Complete_Date | Pending_Date | Cal_Days |
+----------+------------+---------------+--------------+----------+
| 1 | 18/04/2016 | 19/04/2016 | | 1 |
| 2 | 10/04/2016 | 18/04/2016 | 15/04/2016 | 5 |
| 3 | 12/04/2016 | 19/04/2016 | | 7 |
| 4 | 15/04/2016 | 17/04/2016 | 16/04/2016 | 1 |
| 5 | 16/04/2016 | 21/04/2016 | | 5 |
| 1 | 19/04/2016 | 20/04/2016 | | 1 |
| 2 | 20/03/2016 | 31/03/2015 | | |
| 3 | 25/03/2016 | 28/03/2016 | | 3 |
| 4 | 26/03/2016 | 27/03/2016 | | |
| 5 | 27/03/2016 | 30/03/2016 | | 3 |
+----------+------------+---------------+--------------+----------+
Any help would be appreciated. Thanks.

Answer 1

Use JOIN with CASE expression:
SELECT
a.*,
Cal_Days =
DATEDIFF(
DAY,
CASE
WHEN b.DateCol1 = 'Log_Date' THEN a.Log_Date
WHEN b.DateCol1 = 'Complete_Date' THEN a.Complete_Date
ELSE a.Pending_Date
END,
CASE
WHEN b.DateCol2 = 'Log_Date' THEN a.Log_Date
WHEN b.DateCol2 = 'Complete_Date' THEN a.Complete_Date
ELSE a.Pending_Date
END
)
FROM TblA a
INNER JOIN TblB b
ON b.TypeCode = a.TypeCode