SQL same column name allowed

Question

When I run this query in SQL Server

SELECT custid AAA, companyname AAA
FROM Sales.Customers
WHERE country = \'USA\'

It\'s running fine. But

Answer 1

Reason for this problem is:

1st Query

SELECT custid AAA, companyname AAA
   FROM Sales.Customers
   WHERE country = 'USA'

Here, you are assigning column alias at the time of output, so AAA is as column name(alias actually) attached at the time of returning result, but i guess you will see only 1st column with AAA other will be removed because of possible confliction during further reference. so here you are not getting error.

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2nd Query

SELECT * FROM (
    SELECT custid AAA, companyname AAA
    FROM Sales.Customers
    WHERE country = 'USA') BBB

Here you got error because, you are selecting records from Inline View with name BBB, here that Inner Query(inline view) is considered as a Table(for your SELECT * FROM statement), and as we know - basically Table can not have multiple same column name, because of that you are getting error that BBB has multiple AAA column.