Building a snapshot table from audit records - sql-server

Question

I have a Customer table with the following structure.
CustomerId Name Address Phone
1 Joe 123 Main NULL
I also have an Audit table that tracks changes to the Customer table.
Id Entity EntityId Field OldValue NewValue Type AuditDate
1 Customer 1 Name NULL Joe Add 2016-01-01
2 Customer 1 Phone NULL 567-54-3332 Add 2016-01-01
3 Customer 1 Address NULL 456 Centre Add 2016-01-01
4 Customer 1 Address 456 Centre 123 Main Edit 2016-01-02
5 Customer 1 Phone [phone]-1230 Edit 2016-01-03
6 Customer 1 Phone [phone] NULL Delete 2016-01-04
I have a CustomerHistory reporting table that will be populated with a daily ETL job. It has the same fields as Customer Table with additional field SnapShotDate.
I need to write a query that takes the records in Audit table, transforms and inserts into CustomerHistory as seen below.
CustomerId Name Address Phone SnapShotDate
1 Joe 456 Centre 567-54-3332 2016-01-01
1 Joe 123 Main 567-54-3332 2016-01-02
1 Joe 123 Main 843-43-1230 2016-01-03
1 Joe 123 Main NULL 2016-01-04
I guess the solution would involve a self-join on Audit table or a recursive CTE. I would appreciate any help with developing this solution.
Note: Unfortunately, I do not have the option to use triggers or change the Audit table schema. Query performance is not a concern since this will be a nightly ETL process.

Answer 1

You can use below script.
DROP TABLE #tmp
CREATE TABLE #tmp (
id INT Identity
, EntityId INT
, NAME VARCHAR(10)
, Address VARCHAR(100)
, Phone VARCHAR(20)
, Type VARCHAR(10)
, SnapShotDate DATETIME
)
;with cte1 as (
select AuditDate, EntityId, Type, [Name], [Address], [Phone]
from
(select AuditDate, EntityId, Type, Field, NewValue from #Audit) p
pivot
(
max(NewValue)
for Field in ([Name], [Address], [Phone])
) as xx
)
insert into #tmp (EntityId, Name, Address, Phone, Type, SnapShotDate)
select EntityId, Name, Address, Phone, Type, AuditDate
from cte1
-- update NULLs columns with the most recent value
update #tmp
set Name = (select top 1 Name from #tmp tp2
where EntityId = tp2.EntityId and Name is not null
order by id desc)
where Name is null
update #tmp
set Address = (select top 1 Address from #tmp tp2
where EntityId = tp2.EntityId and Address is not null
order by id desc)
where Address is null
update #tmp
set Phone = (select top 1 Phone from #tmp tp2
where EntityId = tp2.EntityId and Phone is not null
order by id desc)
where Phone is null
To Create Test Data, use below script
CREATE TABLE #Customer (
CustomerId INT
, NAME VARCHAR(10)
, Address VARCHAR(100)
, Phone VARCHAR(20)
)
INSERT INTO #Customer
VALUES (1, 'Joe', '123 Main', NULL)
CREATE TABLE #Audit (
Id INT
, Entity VARCHAR(50)
, EntityId INT
, Field VARCHAR(20)
, OldValue VARCHAR(100)
, NewValue VARCHAR(100)
, Type VARCHAR(10)
, AuditDate DATETIME
)
insert into #Audit values
(1, 'Customer', 1, 'Name' ,NULL ,'Joe' ,'Add' ,'2016-01-01'),
(2, 'Customer', 1, 'Phone' ,NULL ,'567-54-3332' ,'Add' ,'2016-01-01'),
(3, 'Customer', 1, 'Address' ,NULL ,'456 Centre' ,'Add' ,'2016-01-01'),
(4, 'Customer', 1, 'Address' ,'456 Centre' ,'123 Main' ,'Edit' ,'2016-01-02'),
(5, 'Customer', 1, 'Phone' ,'567-54-3332' ,'843-43-1230' ,'Edit' ,'2016-01-03'),
(6, 'Customer', 1, 'Phone' ,'843-43-1230' ,NULL ,'Delete' ,'2016-01-04'),
(7, 'Customer', 2, 'Name' ,NULL ,'Peter' ,'Add' ,'2016-01-01'),
(8, 'Customer', 2, 'Phone' ,NULL ,'111-222-3333' ,'Add' ,'2016-01-01'),
(8, 'Customer', 2, 'Address' ,NULL ,'Parthenia' ,'Add' ,'2016-01-01')
Result
EntityId Name Address Phone Type SnapShotDate
1 Joe 456 Centre 567-54-3332 Add 2016-01-01
1 Joe 123 Main 843-43-1230 Edit 2016-01-02
1 Joe 123 Main 843-43-1230 Edit 2016-01-03
1 Joe 123 Main 843-43-1230 Delete 2016-01-04