SQL Unpivot, Cross Apply, Dynamic Query? - sql-server

Question

I'm at crossroads. Can somebody please help me... send me down the right path.
I want to compare / present data from 2 database tables as follows:
Application Database: Many tables have triggers that copy update/delete changes (auditing) to another database.
Audit Database: The information copied from the triggers in the application database
What I want to do should be fairly straightforward. Visually below, is what I want to do to compare data for what changes were made.
I have a working version with CROSS APPLY and UNIONS (it's long and manually typed out for the columns, tables, etc. sucks). The columns are NOT dynamic which makes hundreds of lines of code gross and unmanageable. There has to be a more elegant design. Please any ideas.
I only need to return ONE specific row (ID) from both tables, for comparison.
APP DB
colA colB colC colD
1 hello foo date
APP Audit DB
colA colB colC colD
1 hi bar date
THIS IS WHAT IS WISH TO OUTPUT:
colA_data ColumnName oldData newData
1 colB hi hello
1 colC bar foo
1 colD date date
I hope I have made sense of what I want to accomplish.
I would like to read column names dynamic (not hard), and then put the results side by side like about for reporting reasons. Obviously matching the columns and putting them into rows.
Sample code would be so much appreciated.

Answer 1

Probably the easiest thing to do is using UNPIVOT:
1. Static version
Just to introduce the use of UNPIVOT here is a simple static version that should solve your problem:
declare #appDB table( [colA] int, [colB] nvarchar(50),[colC] nvarchar(50),[colD] nvarchar(50))
declare #auditDB table( [colA] int, [colB] nvarchar(50),[colC] nvarchar(50),[colD] nvarchar(50))
insert into #appDB select 1,'hello', 'foo', 'date'
insert into #auditDB select 1,'hi', 'bar', 'date'
select old.ColA_data, old.ColumnName, old.OldData, new.NewData
from(
select o.colA as ColA_data, o.ColumnName, o.OldData
from #auditDB s
unpivot ([OldData] for [ColumnName] in ([colB], [colC], [colD])) o
) OLD
inner join
(
select n.colA as ColA_data, n.ColumnName, n.NewData
from #appDB t
unpivot ([NewData] for [ColumnName] in ([colB], [colC], [colD])) n
) NEW
on new.ColA_data = old.ColA_data and new.ColumnName = old.ColumnName
Results:
2. Dynamic version
Now the complete version. You can use dynamic SQL to change the columns retrieving them from SQL Server INFORMATION_SCHEMA metadata.
Please note that in this example I added a new column (ColE)
if OBJECT_ID('appDB') is not null drop table appDB
if OBJECT_ID('auditDB') is not null drop table auditDB
create table appDB (colA int, colB nvarchar(50),colC nvarchar(50),colD nvarchar(50),colE nvarchar(50))
create table auditDB(colA int, colB nvarchar(50),colC nvarchar(50),colD nvarchar(50),colE nvarchar(50))
insert into appDB select 1,'hello', 'foo', 'date','time'
insert into auditDB select 1,'hi', 'bar', 'date','time'
declare #cols nvarchar(max)='' --this variable holds all the dates that will become column names
declare #sql nvarchar(max)='' --this variable contains the TSQL dinamically generated
select #cols = #cols + ', [' +COLUMN_NAME + ']'
from INFORMATION_SCHEMA.COLUMNS
where TABLE_NAME='appDB'
and COLUMN_NAME <>'colA'
set #cols = RIGHT(#cols, len(#cols)-2)
set #sql= #sql + ' select old.ColA_data, old.ColumnName, old.OldData, new.NewData '
set #sql= #sql + ' from('
set #sql= #sql + ' select o.colA as ColA_data, o.ColumnName, o.OldData'
set #sql= #sql + ' from auditDB s '
set #sql= #sql + ' unpivot ([OldData] for [ColumnName] in ('+#cols+')) o'
set #sql= #sql + ' ) OLD'
set #sql= #sql + ' inner join'
set #sql= #sql + ' ('
set #sql= #sql + ' select n.colA as ColA_data, n.ColumnName, n.NewData'
set #sql= #sql + ' from appDB t '
set #sql= #sql + ' unpivot ([NewData] for [ColumnName] in ('+#cols+')) n'
set #sql= #sql + ' ) NEW'
set #sql= #sql + ' on new.ColA_data = old.ColA_data and new.ColumnName = old.ColumnName'
exec(#sql)
Results: