How to fill empty cells with previous non empty value?

Question

Hello 
rows where cat=2 do not have price and i want to fill it with previous value wich cat=1 for every item
Table named tb
id        item_no     price         cat 
1           I1             5           1
3           I1                          2          the price must be 5   
4           I1                          2          the price must be 5  too
9           I1             2           1
10         I2             10          1
11         I2             5           1
15         I2              10        1       
28         I2                          2        the price must be 10  
30         I2             2           1
32         I2             10          1
filled table must be like that:
id        item_no     price       cat 
1           I1             5           1
3           I1            
5           2          
4           I1            
5          2          
9           I1             2          1
10         I2             10        1
11         I2             5          1
15         I2             10        1       
28         I2            
10        2      
30         I2             2          1
32         I2             10         1
How can i do that?

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. You have no idea,
do you? 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. Why are you so rude to people? Now we have to guess a everything and try to fix your mess. 
There is no generic “id” in RDBMS; it has to be an industry identifier or “<something in particular>_id”. I like the GTIN. Was that useless vague name supposed to be “<something>_cat” as a category scale? 
CREATE TABLE Items
(foo_seq INTEGER NOT NULL, 
 gtin CHAR(15) NOT NULL, 
 unit_price DECIMAL (12,2), 
 foobar_cat CHAR(1) NOT NULL
     CHECK (foobar_cat IN ('1', '2')), 
CREATE PROCEDURE Add_New_Item
INSERT INTO Items
VALUES
(1, 'I2', 5.00, 1), 
(3, 'I1', NULL, 2), -- unit price must be 5.00
(4, 'I1', NULL, 2), -- unit price must be 5.00
(9, 'I1', 2.00, 1), 
(10, 'I2', 10.00, 1), 
(11, 'I2', 5.00, 1), 
(15, 'I2', 10.00, 1), 
(28, 'I2', NULL, 2), -- unit price must be 10
(30, 'I2', 2.00, 1);
>> rows where cat=2 do not have price and I want to fill it with previous value which cat=1 for every item <<
Rows in a table are not ordered! There is no concept of “previous row” in RDBMS. This is fundamental. Are you using a sequence that you erroneously call “id”? 
But that makes no sense! Look at (1, 'I2', 5.00, 1) and then (3, 'I1', NULL, 2). Why does item 'I2' have anything to do with the price of item 'I1'? 
Can you explain? 
Table named tb
id   item_no   price   cat 
1    'I1',  5    1
3    'I1',  NULL   2  -- the price must be 5   
4    'I1',  NULL   2  -- the price must be 5  too
9    'I1',  2    1
10   'I2',  10   1
11   'I2',  5    1
15   'I2',   10   1   
28   'I2',  NULL   2   -- the price must be 10  
30   'I2',  2    1
32   'I2',  10   1
filled table must be like that:
id   item_no   price   cat 
1    'I1',  5    1
3    'I1',  5    2   
4    'I1',  5   2   
9    'I1',  2   1
10   'I2',  10   1
11   'I2',  5   1
15   'I2',  10   1   
28   'I2',  10   2   
30   'I2',  2   1
32   'I2',  10   1
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