Question

I am using sql server 2008. I have to insert records in temporary table using multiple select statements like below.

Insert into #temp
Select a From TableA

Insert into #temp
Select a From TableB

Insert into #temp
Select a From TableC

Insert into #temp
Select a From TableD

OR

Insert Into #temp
Select A From
(
    Select A from TableA
    Union
    Select B From TableB
    Union
    Select B From TableC
)K

Please advice which approach should be best or any other and Why?

Answer 1

The two techniques you present are not interchangeable. The UNION operation will remove duplicate values while the individual INSERT operations will not. To get identical results, you’d need to use UNION ALL.