Columns into rows with identify in SQL Server - sql-server

Question

I have a question about SQL Server: how to combine three columns into one column with identify in a query.
Table: emp
empid | addr1 | addr2 | addr3
------+-------+-------+------
1 | hyd | chen | pune
2 | del | hyd | nagp
1 | pune | bang | keral
Based on above data I want output like below
empid | empaddrvalues | EmpFields | EmpFieldFlag
------+---------------+-----------+--------------
1 | hyd | addr1 | 1
1 | chen | addr2 | 2
1 | pune | addr3 | 3
2 | del | addr1 | 1
2 | hyd | addr2 | 2
2 | nagp | addr3 | 3
1 | pune | addr1 | 1
1 | bang | addr2 | 2
1 | keral | addr3 | 3
I tried with a query like this:
select
empid,
cast (addr1 as varchar(100)) as empaddrvalues,
'addr1' as EmpFields, '1' as EmpFieldFlag
from
emp
union all
select
empid,
cast (addr2 as varchar(100)) as empaddrvalues,
'addr2' as EmpFields, '2' as EmpFieldFlag
from
emp
union all
select
empid,
cast (addr3 as varchar(100)) as empaddrvalues,
'addr3' as EmpFields, '3' as EmpFieldFlag
from
emp
This query is returning the correct result, but it takes a lot of time due to calling same table 3 times.
Can you please tell me how to write any alternative query to achieve this task in SQL Server?

Answer 1

Unpivot by cross applying with 3 values (1, 2, 3) and CASE on that value to get the corresponding address.
create table dbo.employee
(
empid int,
addr1 varchar(50),
addr2 varchar(50),
addr3 varchar(50)
)
go
insert into dbo.employee(empid, addr1, addr2, addr3)
values
(1, 'hyd', 'chen', 'pune'),
(2, 'del', 'hyd', 'nagp'),
(1, 'pune', 'bang', 'keral');
go
select
e.empid,
case nums.n when 1 then addr1 when 2 then addr2 when 3 then addr3 end as empaddrvalues,
--concat('???', nums.n) as xyz,
nums.n as EmpFieldFlag
from dbo.employee as e
cross apply (values(1), (2), (3)) as nums(n);
go
drop table dbo.employee
go