MySQL: Combining two repetitive queries

I have this in info in a table where sometime the pd_id is NULL

doc_id  doc_title   doc_order   pg_id 
14      a.zip       1           NULL    
15      b.zip       2           NULL    
12      c.zip       1           16  
13      d.zip       2           16      
3       f.doc       3           16      
4       g.doc       4           16  

When I want to query the item with the pg_id 16, I do this,

SELECT * 
FROM root_documents 
WHERE root_documents.pg_id = '16'

While when I want query the item without any pd_id, I do this,

SELECT * 
FROM root_documents 
WHERE root开发者_C百科_documents.pg_id IS NULL

I find two queries are repetitive, so I try to this,

SELECT * 
FROM root_documents 
WHERE (root_documents.pg_id = ? OR root_documents.pg_id IS NULL)

So when I want to get this result only when I query the item with pg_id 16,

doc_id  doc_title   doc_order   pg_id 
12      c.zip       1           16  
13      d.zip       2           16      
3       f.doc       3           16      
4       g.doc       4           16

But I get all of them intead!

doc_id  doc_title   doc_order   pg_id 
14      a.zip       1           NULL    
15      b.zip       2           NULL    
12      c.zip       1           16  
13      d.zip       2           16      
3       f.doc       3           16      
4       g.doc       4           16

How can I fix this query,

SELECT * 
FROM root_documents 
WHERE (root_documents.pg_id = ? OR root_documents.pg_id IS NULL)

Or do I have to repeat the query like I usually do?

EDIT:

This answer looks strange to me when I have a parameter to be passed into the placeholder:

SELECT * FROM root_documents WHERE (root_documents.pg_id = '16' OR (root_documents.pg_id IS NULL AND '16' IS NULL))

and when without a parameter,

SELECT * FROM root_documents WHERE (root_documents.pg_id = NULL OR (root_documents.pg_id IS NULL AND NULL IS NULL))

Note that I pass NULL into the placeholder when no value is returned.

Sorry : you are using MYSQL... Disregard.

One way is this:

SELECT * 
FROM root_documents 
WHERE isnull(root_documents.pg_id, -1) = ?

-1 or any value you never expect to find in [root_documents.pg_id]. Then when you want to search for NULL query for that value.

I think you want something like this:

SELECT * 
FROM root_documents 
WHERE (root_documents.pg_id = ? OR (root_documents.pg_id IS NULL AND ? IS NULL))

You could set the ? to a number when you want that pg_id, or to the string IS NULL when you want the ones that are null.

I think the following should work for you(updated):

SELECT * 
FROM root_documents r1
WHERE (r1.pg_id = ? 
OR (NOT EXISTS 
 (SELECT * FROM FROM root_documents r2 
  WHERE r2.pg_id = ? )
 AND r1.pg_id IS NULL)