Conditional FROM clause - sql-server

Question

My coworkers are using entity framework and have got 3 (schematically) identical databases. These databases are updated and modified by their application. I am writing another, separate application to gather information about their application.
I am trying to use stored procedures but having trouble. It seems I must have three copies of my query in every stored procedure (one for each database) and JOIN them all at the end. I don't want to have three copies of every query with only the table name changed. Can I specify using a parameter, CASE statement, or something else the table I use in my FROM Clause?

Answer 1

Two options: dynamic SQL, or a UNION ALL statement.
SELECT columnlist
FROM TABLE1
WHERE #param = 'Table1'
UNION ALL
SELECT columnlist
FROM TABLE2
WHERE #param = 'Table2'
UNION ALL
SELECT columnlist
FROM TABLE3
WHERE #param = 'Table3'

Answer 2

Since you are working with stored procedures, you can pass the table name from which you want to query as parameter like
create procedure sp_test
#tab_name varchar(10)
as
begin
if(#tab_name = 'Table1')
select * from Table1
else if (#tab_name = 'Table2')
select * from Table2
else
select * from Table3
end
Then run your SP like
exec sp_test 'Table1'
EDIT:
As per your comment you want to change the DB name in your query. So in DB.HistoryOne JOIN DB.HistoryTwo you want to change the DB to DB1. You can do it like below in a procedure
create procedure sp_DB_change
#DBname varchar(10)
as
begin
declare #sql varchar(200);
set #sql = 'SELECT AVG(DATEDIFF(s, StartDate, OtherStartDate)) AS time1 ,
CAST(OtherStartDate AS Date) AS [Date]
FROM DB.HistoryOne
JOIN DB.HistoryTwo ON HistoryOne.Id = HistoryTwo.Id
WHERE StartDate IS NOT NULL
AND OtherStartDate IS NOT NULL
AND OtherStartDate > DATEADD(d, -7, GETDATE())
GROUP BY CAST(OtherStartDate AS DATE)';
select #sql = REPLACE(#sql,'DB',#newdb)
exec (#sql)
end
Then run your SP like
exec sp_DB_change 'testDB'
So your original query
SELECT AVG(DATEDIFF(s, StartDate, OtherStartDate)) AS time1 ,
CAST(OtherStartDate AS Date) AS [Date]
FROM DB.HistoryOne
JOIN DB.HistoryTwo ON HistoryOne.Id = HistoryTwo.Id
WHERE StartDate IS NOT NULL
AND OtherStartDate IS NOT NULL
AND OtherStartDate > DATEADD(d, -7, GETDATE())
GROUP BY CAST(OtherStartDate AS DATE)
Will be converted to
SELECT AVG(DATEDIFF(s, StartDate, OtherStartDate)) AS time1 ,
CAST(OtherStartDate AS Date) AS [Date]
FROM testDB.HistoryOne
JOIN testDB.HistoryTwo ON HistoryOne.Id = HistoryTwo.Id
WHERE StartDate IS NOT NULL
AND OtherStartDate IS NOT NULL
AND OtherStartDate > DATEADD(d, -7, GETDATE())
GROUP BY CAST(OtherStartDate AS DATE)