Why does "SELECT DISTINCT a, b FROM..." return fewer records than "SELECT DISTINCT A + '|' + B FROM..."?

I have a query that's selecting a bunch of fields related to names and addresses of customers but it boils down to:

SELECT DISTINCT a, b, c, ... FROM big_dumb_flat_table

it returns a bunch of records (10986590). When I replace the commas in the select-list to format it as a pipe-separated concatenated string:

SELECT DISTINCT a + '|' + b + '|' + c + '|' + ... FROM big_dumb_flat_table

it's returning 248 more records. I've reassured myself that there are no pipes in any of the fields that could开发者_StackOverflow社区 be screwing the fidelity of the returned set. What's going on here?

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Trailing spaces could cause this. For string comparisons these are ignored.

CREATE TABLE #T
(
a varchar(10),
b varchar(10),
c varchar(10)
)

INSERT INTO #T
SELECT 'a ' as a, 'b' as b, 'c ' as c union all
SELECT 'a' as a, 'b' as b, 'c ' as c

SELECT DISTINCT a, b, c  
FROM #T /*1 result*/

SELECT DISTINCT a + '|' + b + '|' + c + '|'   
FROM #T /*2 results*/


SELECT DISTINCT LTRIM(RTRIM(a)) + '|' + LTRIM(RTRIM(b)) + '|' +
                LTRIM(RTRIM(c)) + '|'   
FROM #T /*1 result*/

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The really aren't any scenarios that I can think of that would get you MORE records, only fewer. I would simplify the query by only selecting a + '|', then add more columns as you go.