Can you make a key with delete cascade when there is no primary key?

Question

I have a table with a primary key, and also a field called 'tpid', which is sometimes null, and sometimes has an integer value in it. This integer value corresponds a primary key in another table, and that key is also called tpid.
I would like the following to happen.
If a row in table 1 is deleted, and it has a tpid that is not null, then I want the corresponding row in the second table to be deleted too.
I think a trigger could be written to do this, but it would be better if a foreign key constraint with cascade delete could be used.
One reason that is better is that whenever I script out my database, triggers seem to be left out by default, unless I specifically ask that they not be..
So can this be done - where column in table 1 has some nulls, and is not a primary key, but when its deleted, you want to delete a matching row in a table where it IS a primary key.
Thanks,
Gideon

Answer 1

Please post DDL, so that people do not have to guess what the keys, constraints, Declarative Referential Integrity, data types, etc. in your schema are. Learn how to follow ISO-11179 data element naming conventions and formatting rules. Temporal data should
use ISO-8601 formats. Code should be in Standard SQL as much as possible and not local dialect. 
This is minimal polite behavior on SQL forums. 
>> I have a table with a PRIMARY KEY, <<
That is redundant. Table has a key by definition, but the fact that you said it this way tells us you have a high level of ignorance. 
>> .. and also a field [sic] called “tp_id”, which is sometimes NULL, and sometimes has an integer value in it. <<
Yep! Wrong again! A column is not anything like a field. The only place “field” is use in SQL is for {year, month, day, hour, minute, second} in temporal values. 
>> This integer value corresponds a PRIMARY KEY in another table, and that key is also called tp_id. <<
NO, NO, NO! A key is never NULL by definition. It cannot be NULL in your un-named second table. Where is the DDL? Why do you have manners of a pig? 
I will guess, since we have no clear specs from you, that you want to replace the NULL with a dummy value that can be used with DRI actions. 
Would you like to give us enough information to help you? 
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