Compare two tables and find differences in column values - sql

Question

I want to compare two tables (identical columns) and find out that what column value changed.
Here is an example with sample data.
employee_original table has 6 columns.
CREATE TABLE [dbo].[employee_original](
[emp_id] [int] IDENTITY(1,1) NOT NULL,
[first_name] [varchar](100) NOT NULL,
[last_name] [varchar](100) NOT NULL,
[salary] int NOT NULL,
[city] [varchar](20) NOT NULL,
[department] [varchar](20) NOT NULL,
PRIMARY KEY CLUSTERED
(
[emp_id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 90) ON [PRIMARY]
) ON [PRIMARY]
GO
INSERT INTO employee_original VALUES ( 'Julia', 'Schultz', 100, 'New York', 'Tech');
INSERT INTO employee_original VALUES ( 'Vincent', 'Trantow', 200, 'Moscow', 'HR');
INSERT INTO employee_original VALUES ( 'Whitney ', 'Pouros', 500, 'Miami', 'Accounting');
INSERT INTO employee_original VALUES ( 'Chandler', 'Osinski', 10, 'Singapore', 'Purchasing');
INSERT INTO employee_original VALUES ( 'Sydnie', 'Green', 700, 'Ireland', 'Operations');
INSERT INTO employee_original VALUES ( 'Josefa', 'Anderson', 800, 'Berlin', 'Purchase');
INSERT INTO employee_original VALUES ( 'Brayan', 'Bergstrom', 900, 'New York', 'Operations');
INSERT INTO employee_original VALUES ( 'Shyanne', 'Kris', 900, 'New York', 'Sales');
employee_modified has same employee but some of the attributes have changed for few employees.
CREATE TABLE [dbo].[employee_modified](
[emp_id] [int] IDENTITY(1,1) NOT NULL,
[first_name] [varchar](100) NOT NULL,
[last_name] [varchar](100) NOT NULL,
[salary] int NOT NULL,
[city] [varchar](20) NOT NULL,
[department] [varchar](20) NOT NULL,
PRIMARY KEY CLUSTERED
(
[emp_id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 90) ON [PRIMARY]
) ON [PRIMARY]
GO
INSERT INTO employee_modified VALUES ( 'Julia', 'Schultz', 100, 'New York', 'Tech');
INSERT INTO employee_modified VALUES ( 'Vincent', 'Wyman', 500, 'Moscow', 'HR');
INSERT INTO employee_modified VALUES ( 'Whitney ', 'Pouros', 500, 'Miami', 'Sales');
INSERT INTO employee_modified VALUES ( 'Chandler', 'Osinski', 10, 'Singapore', 'Purchasing');
INSERT INTO employee_modified VALUES ( 'Sydnie', ' Cartwright', 900, 'Ireland', 'Operations');
INSERT INTO employee_modified VALUES ( 'Joseph', 'Anderson', 800, 'Berlin', 'Purchase');
INSERT INTO employee_modified VALUES ( 'Bryan', 'Bergstrom', 900, 'Naples', 'Operations');
INSERT INTO employee_modified VALUES ( 'Shyanne', 'Jakubowski', 900, 'New York', 'Accounting');
I am looking for a result that can tell me what field changed for which employee.
e.g. emp_id =2 has last name and salary change. So output should look like:
emp_id attribute orignial_value new_value
2 last_name Trantow Wyman
2 salary 200 500
This is what I have tried so far:
(1) Join tables and find what changed :
DROP TABLE IF EXISTS #temp;
SELECT distinct
o.emp_id,
o.first_name [original_first_name], m.first_name [modified_first_name],
o.last_name [original_last_name], m.last_name [modified_last_name],
o.salary [original_salary], m.salary [modified_salary],
o.city [original_city], m.city [modified_city],
o.department [original_department], m.department [modified_department]
into #temp from
[dbo].[employee_original] o inner join [dbo].[employee_modified] m on o.emp_id = m.emp_id
select * from #temp
Gives me
(2) Self join with #temp and find out what attribute has changed.
-- All Last Name Changes.
select distinct t1.emp_id, t1.original_last_name, t2.modified_last_name
from #temp t1
inner join #temp t2 on t1.emp_id = t2.emp_id
where t1.original_last_name <> t2.modified_last_name
-- All Department changes
select distinct t1.emp_id, t1.original_department, t2.modified_department
from #temp t1
inner join #temp t2 on t1.emp_id = t2.emp_id
where t1.original_department <> t2.modified_department
Any pointers on how I can get to my desired result.

Answer 1

Here is an option that will dynamically unpivot your data without actually using dynamic SQL.
Example
Select emp_id
,[key]
,Org_Value = max( case when Src=1 then Value end)
,New_Value = max( case when Src=2 then Value end)
From (
Select Src=1
,emp_id
,B.*
From [employee_original] A
Cross Apply ( Select [Key]
,Value
From OpenJson( (Select A.* For JSON Path,Without_Array_Wrapper,INCLUDE_NULL_VALUES ) )
) B
Union All
Select Src=2
,emp_id
,B.*
From [employee_modified] A
Cross Apply ( Select [Key]
,Value
From OpenJson( (Select A.* For JSON Path,Without_Array_Wrapper,INCLUDE_NULL_VALUES ) )
) B
) A
Group By emp_id,[key]
Having max( case when Src=1 then Value end)
<> max( case when Src=2 then Value end)
Order By emp_id,[key]
Results

Answer 2

You can use the following code to unpivot all possible changes
SELECT
o.emp_id,
v.column_name,
v.old_value,
v.new_value
FROM employee_original o
JOIN employee_modified m ON o.emp_id = m.emp_id
CROSS APPLY (
SELECT 'first_name', CAST(o.first_name AS nvarchar(max)), CAST(m.first_name AS nvarchar(max))
WHERE o.first_name <> m.first_name
UNION ALL
SELECT 'last_name', o.last_name, m.last_name
WHERE o.last_name <> m.last_name
UNION ALL
SELECT 'salary', o.salary, m.salary
WHERE o.salary <> m.salary
UNION ALL
SELECT 'city', o.city, m.city
WHERE o.city <> m.city
UNION ALL
SELECT 'department', o.department, m.department
WHERE o.department <> m.department
) v(column_name, old_value, new_value);