通过不检测重复但有欺骗来分组。 奇怪 SQL 服务器 - Azure Synapse 数据库专用 SQL 池

Question

I have encountered a strange (until I understand the logical reason) behaviour of group by in a SQL Server database. There are many duplicates in a table, for which when I query I get duplicate rows but when I try to find all dupes using group by or row_number strategy I get 0 records. But when I add "Cast" to the group by / row_number I get correct list of duplicates.

The datatype is nvarchar for all 3 keys.
Can someone tell me why this is happening?

Added the query and its output

select top 10 len(VBELN) len_vblen, len(MANDT) , len(posnr) ,  * from [SRC_SAP_R3].[LIPS] where VBELN = '6316785926'

select cast(MANDT as nvarchar) as "MANDT",cast(VBELN as nvarchar) as "VBELN" , cast(posnr as nvarchar) as "posnr", count(*) from [SRC_SAP_R3].[LIPS]
group by cast(MANDT as nvarchar),cast(VBELN as nvarchar)  , cast(posnr as nvarchar) 
having count(*)>1;

select cast(MANDT as varchar) as "MANDT",cast(VBELN as varchar) as "VBELN" , cast(posnr as varchar) as "posnr", count(*) from [SRC_SAP_R3].[LIPS]
group by cast(MANDT as varchar),cast(VBELN as varchar)  , cast(posnr as varchar) 
having count(*)>1;

select MANDT, VBELN ,posnr, count(1) from [SRC_SAP_R3].[LIPS]
group by MANDT, VBELN ,posnr
having count(1)>1;

Answer 1

I tried to repro this in Azure Synapse Analytics. As @Martin Smith said the len() function will ignore the trailing spaces while computing the total length of the column. When I tried with datalength() function, the length of trailing spaces is also included. Below is the repro.

• A table is created with varchar column and one data is inserted with trail spaces and other data is without spaces.

 create  table SAP_TAB (VBELN varchar(100))
insert  into SAP_TAB values('500 ')
insert  into SAP_TAB values('500')

• Then len() function, datalength() function is applied to data. Also, casted the data as varchar and length function is applied to casted data. Below is the query.

select VBELN,len(VBELN) as [length_VBELN],
datalength(VBELN) as [data_length_VBELN],
len(cast(VBELN as  varchar(10))) as 
[length_varchar_casted_VBELN]
from sap_tab

Result

VBELN	length_VBELN	data_length_VBELN	length_varchar_casted_VBELN
500	3	5	3
500	3	3	3