Conditional string concatenation in PostgreSQL I have a table parcels which currently contains the columns owner_addr1, owner_addr2, owner_addr3. Sometimes, one or both of the latter two fields is empty. I want to combine them into a single new field, owner_addr where each of the above fields is concatenated with // between each of them. 

But if one or more of the original columns is NULL, I don't want to concatenate // to the result column. So for instance, if owner_addr1 is 123 4th Avenue SE and owner_addr2 and owner_addr3 are NULL, then I want the result column to just be 123 4th Avenue SE, not 123 4th Avenue SE // // (which would happen if I just did CONCAT() with // between the NULL strings ... I only want to add // between non-NULL columns, or leave it out altogether if there is only one non-NULL column.

Is there an easy way to do this kind of conditional concatenation in Postgresql, where it leaves out empty lines? Or should I write a python script to do this?
The function concat_ws() does exactly what you want. The first parameter is used as glue between the rest. Nulls are ignored:

select concat_ws('//', owner_addr1, owner_addr2, owner_addr3)


Test:

red=# select concat_ws('//', 'abc', null, null, 'xx', null, 'xyz', null) 
          as address;
   address    
--------------
 abc//xx//xyz
(1 row)