How to replace NULL with the previous value?

Question

My table:

field5	field19
Dr	1
NULL	2
NULL	3
Td	4
Td	5
NULL	6

SQL query should give following result:

field5	field19
Dr	1
Dr	2
Dr	3
Td	4
Td	5
Td	6

Answer 1

Window function would help :

select *,  
      max(field5) over (order by field19)
from table t;

You can also use correlated subquery :

select t.*,
       (select t1.field5 
        from table t1 
        where t1.field19 <= t.field19 and t1.field5 is not null
        order by t1.field19 desc 
        limit 1
       )
from table t;