为什么"SELECT DISTINCT a,b FROM ..."返回的记录少于"SELECT DISTINCT A +'|' + B FROM ......"？

Question

我有一个查询,它选择了一系列与客户名称和地址相关的字段,但归结为:

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

它返回一堆记录(10986590).当我替换select-list中的逗号以将其格式化为管道分隔的连接字符串时:

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

它返回了248条记录.我已经向自己保证,任何一个领域都没有管道可能会破坏返回集合的保真度.这里发生了什么？

Answer 1

尾随空格可能会导致此问题.对于字符串比较,这些将被忽略.

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*/