Why SingleOrDefault result TOP(2) in SQL?

Solution 1:

It selects top 2 so that if there are actually 2 or more than 2 records in the database, an exception would be thrown. If it only selects top 1 there would be no way to error out.

Solution 2:

By asking for the SingleOrDefault of a sequence, you are asking for this behaviour:

• if the sequence has exactly 0 elements, return the default for the sequence's element type
• if the sequence has exactly 1 element, return the element
• if the sequence has more than 1 element, throw

Doing a TOP (1) would empower the first two parts of this, but not the third. Only by doing a TOP (2) can we differentiate between exactly 1 record and more than 1 record.

If you don't want or need the third part of the above behviour, instead use FirstOrDefault.