Filter Data in SQL Server - sql

Question

I have a table like below :
[TMP_TBL]
(
...
[DT_REFUND] [varchar(20)] NULL,
[MSG_PROCESS] [nvarchar(max)] NULL,
...
)
and it contains data like this:
| DT_REFUND | MSG_PROCESS |
| A-BC---D | WRONG DATETYPE |
| 20180301 | NULL |
| 20180331 | NULL |
When I do a query like:
select *
from TMP_TBL
where isnull(MSG_PROCESS, '') = ''
everything goes fine.
But when I do query like:
select *
from TMP_TBL
where isnull(MSG_PROCESS, '') = ''
and convert(DATE, DT_REFUND) >= convert(DATE, '20180301');
I get an error:
Conversion failed when converting date and/or time from character string
Why did this happen, and how to fix it?
Thanks

Answer 1

Ideally you should have avoided using VARCHAR data type for dates.
For the current scenario, you can use TRY_CONVERT like following.
select *
from TMP_TBL
where isnull(MSG_PROCESS, '') = ''
and TRY_CONVERT(DATE, DT_REFUND) >= convert(DATE, '20180301');
As your date column is VARCHAR, so there is a possibility of having empty spaces also, to handle this you can try RTRIM function before converting to date like following.
TRY_CONVERT(DATE, RTRIM(DT_REFUND)) >= convert(DATE, '20180301');

Answer 2

First you check the value in the column is DATE format or other format, using ISDATE function. And compare with the date value.
If it is not in date format it returns NULL value.
Try this:
SELECT *
FROM TMP_TBL
WHERE ISNULL(MSG_PROCESS,'') = ''
AND CONVERT(DATE, CASE WHEN ISDATE(DT_REFUND)=1 THEN DT_REFUND ELSE NULL END) >= CONVERT(DATE, '20180301');

Answer 3

You just need to filter out null values and compared date via cast() or convert() function
select *
from TMP_TBL
where MSG_PROCESS is null and
cast(DT_REFUND as date) >= '20180301';
This assumes null value has always date as your sample data suggests
In other way, first you could check DT_REFUND has some kind of date format data via TRY_CONVERT() function. So, your where clause should do
where MSG_PROCESS is null and TRY_CONVERT(date, DT_REFUND) is not null and
cast(DT_REFUND as date) >= '20180301';
However, the above has some redundant filtration before cast so you could also express that as
where MSG_PROCESS is null and TRY_CONVERT(date, DT_REFUND) >= '20180301';