Data Type

[lahuva]

I am new to Oracle so please be kind if this is a dumb question or going about this all wrong.

I have to work with a field A that is used for different types of data depending on another field B's number. A contains a date or at least it looks like a date when B is for example equal to 1. I limited the query to only cases where B=1 and for some records when I use to_date(A) it converts it to a date, however some records apparently contain data that is not a date and then it errors out with a message that the field contains an inconsisent data type Also I cannot criteria on to_date(A) which is really what I want to do because it errors out with a not a valid month error and sometimes I get a fetch out of sequence message (whatever that means)
.
Is there a way similar to the nvl for null to 0 to convert the entries that are not convertable to date to a fake date say 1/1/1800 and then I could query on the date?

Below is one of my various attempts

[code]
select * --nvl(to_date(prju_data), 0)
from keyowner.PRJU_PROJECT_USER_COLUMN, keyowner.prj_project
where prju_prj_project_num=prj_project_num
and prju_prjc_number =10000007099404
and prj_status='D'
and to_date(prju_data)> to_date('01/04/2007', 'dd/mm/yyyy')
[code]

Sometimes I included the date criteria and sometimes I used to_date in the select. I did not always link to the other table either, but I will need to.