Display the difference between rows in the same table - sql

Question

I have a table named Employee_audit with following schema,
emp_audit_id
eid
name
salary
...
1
1
Daniel
1000
...
2
1
Dani
1000
...
3
1
Danny
3000
...
My goal is to write a SQL query which will return in following format, considering the first row also as changed value from null.
columnName
oldValue
newValue
name
null
Daniel
salary
null
1000
name
Daniel
Dani
name
Dani
Danny
salary
1000
3000
...
...
...
Finally reached to below solution
CREATE TABLE Employee_audit (
emp_audit_id int,
eid int,
name varchar(50),
salary int,
department varchar(50)
)
insert into Employee_audit (emp_audit_id, eid, name, salary,department)
values
(1, 1, 'Daniel', 1000,'ROP'),
(2, 1, 'Dani', 1000,'ROP'),
(3, 1, 'Danny', 3000,'ROP');
with diffs as (
select 'name' colName, emp_audit_id, eid, lag(name, 1, null) over (partition by eid order by emp_audit_id) oldValue, name newValue
from Employee_audit
union all
select 'salary', emp_audit_id, eid, cast(lag(salary, 1, null) over (partition by eid order by emp_audit_id) as varchar), cast(salary as varchar) newValue
from Employee_audit
union all
...
)
select *
from diffs
where oldValue <> newValue or oldValue is null
order by emp_audit_id, eid
Returns:
emp_audit_id
columnName
oldValue
newValue
1
name
NULL
Daniel
1
salary
NULL
1000.00
2
name
Daniel
Dani
3
name
Dani
Danny
3
salary
1000.00
3000.00
...
...
...
...
But the problem is, the query is very slow because to track 10 fields we have to write 10 union all.
How can I more optimize the query like in a single scan how can I do it?

Answer 1

Here is an option that will dynamically unpivot your data without actually using Dynamic SQL.
Example
;with cte as (
Select emp_audit_id
,eid
,[key]
,newValue=[value]
,oldvalue = lag(value) over (partition by eid,[key] order by emp_audit_id)
From Employee_audit A
Cross Apply ( Select [key],[value] From OpenJson( (Select A.* For JSON Path,Without_Array_Wrapper ) ) ) B
Where [key] not in ('emp_audit_id','eid')
)
Select emp_audit_id
,columName = [key]
,oldvalue
,newvalue
From cte
Where newvalue<>oldvalue or oldvalue is null
Returns
emp_audit_id columName oldvalue newvalue
1 department NULL ROP
1 name NULL Daniel
2 name Daniel Dani
3 name Dani Danny
1 salary NULL 1000
3 salary 1000 3000

Answer 2

I would use apply:
select t.emp_audit_id, v.columnName, v.newValue,
lag(v.newValue) over (partition by eid, columnName order by emp_audit_id) as oldValue
from some_table t cross apply
(values ('name', t.name),
('salary', t.salary),
. . .
) v (columnName, newValue);
If you need to cast the values so they are strings, that goes in the values clause:
select t.emp_audit_id, v.columnName, v.newValue,
lag(v.newValue) over (partition by eid, ColumnName order by emp_audit_id) as oldValue
from some_table t cross apply
(values ('name', t.name),
('salary', cast(t.salary as varchar(255))),
. . .
) v (columnName, newValue);