Question

I have these two tables in mysql:

create table a (
    id INT NOT NULL,
    valueA INT
);

create table b (
    id INT NOT NULL,
    valueB INT,
    priority INT
);

Example data – table a:

1, 30  
2, 35  
3, NULL  
4, 8  
5, 50  
6, NULL  
7, 11  
8, NULL  

Example data – table b:

1, 100, 1  
2, 200, 2  
3, 250, 3  
4, 350, 4  

And I want result:

30  
35  
100  
8  
50  
200  
11  
250  

I want select a.valueA. If in a.valueA is a NULL value I want to replace it with b.valueB. The first occurrence of NULL in a.valueA I want to replace with b.valueB where priority = 1, the second occurrence of NULL in a.valueA I want to replace with b.valueB where priority = 2 and so on.

Please guys, don´t you have any idea how to solve this select in mysql?

Thanks.

Answer 1

Is this what you want? :

SELECT a1.id,
       COALESCE
       ( a1.valueA,
         ( SELECT b.valueB
             FROM b
            WHERE b.priority =
                   1 + ( SELECT COUNT(1)
                           FROM a AS a2
                          WHERE a2.id < a1.id
                            AND a2.valueA IS NULL
                       )
         )
       )
  FROM a AS a1
 ORDER BY a1.id
;