How to write dynamic query columns into rows in SQL Server?

问题

I have a question about in SQL Server: how to combine 4 columns into 1 column in a query.

Table: emp

empid | addr | sal | doj
------+------+-----+------------
  1   | hyd  | 10  | 10-01-1990
  2   | del  | 20  | 12-03-1999
  1   | pune | 50  | 12-03-2017

Based on above data I want output like below

empid | empvalues
------+---------------
1     | hyd
1     | 10
1     | 10-01-1990
2     | del
2     | 20
2     | 12-03-1999
1     | pune
1     | 50
1     | 12-03-2017

I tried with a query like this:

select 
    empid, cast (addr as varchar(100)) as empvalues 
from emp

union all

select empid, cast (sal as varchar(100)) as empvalues 
from emp

union all

select empid, cast (doj as varchar(100)) as empvalues 
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?

回答1:

You could try using UNPIVOT, like this...

SELECT empid, empvalues
FROM (
    select empid, addr, convert(varchar(100), sal) as sal,  convert(varchar(100), doj, 103) as doj
    from emp
) pv
UNPIVOT
(
    empvalues
    FOR ev in (addr, sal, doj)
) AS ev

produces this output...

Resources for your reference...

https://codingsight.com/understanding-pivot-unpivot-and-reverse-pivot-statements/ https://docs.microsoft.com/en-us/sql/t-sql/queries/from-using-pivot-and-unpivot?view=sql-server-ver15

回答2:

Use Cross Apply:

create table emp (empid int, addr varchar(10), sal int, doj datetime) 

insert into emp (empid, addr, sal, doj)
values (1, 'hyd', '10', '10-01-1990'),
  (2, 'del', '20', '12-03-1999'),
  (1, 'pune', '50', '12-03-2017'); 


select empid,empvalues from emp
cross apply (values (cast (addr as varchar(10)))
,(cast (sal as varchar(10)))  
,(Format(doj,'MM-dd-yyyy')))   d(empvalues)

Results:

empid   empvalues
1   hyd
1   10
1   10-01-1990
2   del
2   20
2   12-03-1999
1   pune
1   50
1   12-03-2017