SQL JOIN combining columns to a single column - sql

Question

I've written the following query for Microsoft SQL Server 2008 R2 ...
with
downloads as
(
select convert(varchar(10), timestamp, 112) as downloadDate, COUNT(*) as counter
from <download_table>
group by convert(varchar(10), timestamp,112)
),
uploads as
(
select CONVERT(varchar(10), dateadded, 112) as uploadDate, COUNT(*) as counter
from <upload_table>
group by CONVERT(varchar(10), dateadded, 112)
)
select
downloads.downloadDate,
uploads.uploadDate,
downloads.counter as dCount,
uploads.counter as uCount
from downloads
full join uploads on uploads.uploadDate = downloads.downloadDate
order by downloadDate desc;
which returns the following table...
downloadDate uploadDate dCount uCount
20121211 NULL 40 NULL
20121210 NULL 238 NULL
20121207 20121207 526 4
20121206 20121206 217 12
20121205 NULL 108 NULL
20121204 20121204 190 13
20121203 NULL 141 NULL
20121130 20121130 248 187
20121129 NULL 134 NULL
20121128 NULL 102 NULL
20121127 20121127 494 57
20121126 NULL 153 NULL
20121119 20121119 319 20
20121118 NULL 4 NULL
20121116 20121116 215 16
20121112 20121112 431 144
20121109 20121109 168 48
20121108 20121108 132 181
NULL 20121125 NULL 3
but I can't get the two dates to combine into a single 'date' column without getting some NULL entries, nor can I get the NULL values in the dCount or uCount to display 0 instead of NULL.
Can somebody help me with this please ?

Answer 1

In SQL Serve,r you can use COALESCE around the date field which returns the first non-null value and ISNULL around the count totals to replace the null value with zero:
with
downloads as
(
select convert(varchar(10), timestamp, 112) as downloadDate, COUNT(*) as counter
from download_table
group by convert(varchar(10), timestamp,112)
),
uploads as
(
select CONVERT(varchar(10), dateadded, 112) as uploadDate, COUNT(*) as counter
from upload_table
group by CONVERT(varchar(10), dateadded, 112)
)
select
coalesce(downloads.downloadDate, uploads.uploadDate) as dDate,
isnull(downloads.counter, 0) as dCount,
isnull(uploads.counter, 0) as uCount
from downloads
full join uploads
on uploads.uploadDate = downloads.downloadDate
order by downloadDate desc;
See SQL Fiddle with Demo
Result:
| DDATE | DCOUNT | UCOUNT |
------------------------------
| 20121211 | 2 | 0 |
| 20121210 | 1 | 1 |
| 20121207 | 1 | 0 |
| 20121206 | 2 | 1 |
| 20121208 | 0 | 1 |
| 20121209 | 0 | 1 |
| 20121204 | 0 | 1 |
| 20121205 | 0 | 1 |

Answer 2

Depending on your SQL dialect, something like IFNULL(), NVL(), COALESCE(), IIF() etc. will help you get rid of the NULLs in favour of a date in the past, such as '18000101'.
After having done that, you can use MAX(), SWITCH(), IIF(), IF() or friends to make a single "last usage date" column.

Answer 3

You can use coalesce and nvl like this:
with
downloads as
(
select convert(varchar(10), timestamp, 112) as downloadDate, COUNT(*) as counter
from <download_table>
group by convert(varchar(10), timestamp,112)
),
uploads as
(
select CONVERT(varchar(10), dateadded, 112) as uploadDate, COUNT(*) as counter
from <upload_table>
group by CONVERT(varchar(10), dateadded, 112)
)
select
coalesce(downloads.downloadDate, uploads.uploadDate) as dDate,
nvl(downloads.counter, 0) as dCount,
nvl(uploads.counter, 0) as uCount
from downloads
full join uploads on uploads.uploadDate = downloads.downloadDate
order by downloadDate desc;

Answer 4

Firstly, it is much better to cast a datetime to a date if you want to remove the time element, rather than converting to varchar, in SQL-Server 2008 you can simply use:
CAST(DateAdded AS DATE)
Then rather than using a FULL JOIN I would do this do this using UNION ALL, it should perform better (although I can't say 100% without testing on your actual data).
WITH Data AS
( SELECT [Date] = CAST(Timestamp AS DATE),
[Downloads] = 1,
[Uploads] = 0
FROM Download_Table
UNION ALL
SELECT [Date] = CAST(DateAdded AS DATE),
[Downloads] = 0,
[Uploads] = 1
FROM Upload_Table
)
SELECT [Date],
[Downloads] = SUM(Downloads),
[Uploads] = SUM(Uploads)
FROM Data
GROUP BY [Date]
ORDER BY [Date];