MySQL get rows but prefer one column value over another

Question

A bit of a strange one, I want to write a MySQL query that will get results from a table, but prefer one value of a column over another, ie

id   name    value            


        

Answer 1

If I understand correctly, you want the value of a name given a specific priority, or the value associated with a NULL priority. (You do not necessarily want the MAX(priority) that exists.)

Yes, you've got some awkward design issues which you should address, but let's solve the problem you do have at present (and you can later migrate to the problem you ought to have :) ):

mysql> SET @priority = 1;  -- the priority we want, if recorded

mysql> PREPARE stmt FROM "
       SELECT
         t0.*
       FROM
         t t0
       LEFT JOIN
         (SELECT DISTINCT name, priority FROM t WHERE priority = ?) t1
           ON t0.name = t1.name
       WHERE
         t0.priority = t1.priority
           OR
         t1.priority IS NULL
       ";

mysql> EXECUTE stmt USING @priority;
+----+-------+--------+----------+
| id | name  | value  | priority |
+----+-------+--------+----------+
|  2 | name1 | valueX |        1 | 
|  3 | name2 | value2 |     NULL | 
|  4 | name3 | value3 |     NULL | 
+----+-------+--------+----------+
3 rows in set (0.00 sec)

(Note that I changed the prioritized value of "name1" to "valueX" in the above -- your original formulation had identical value values for "name1" regardless of priority, which made it hard for me to understand why you cared to discriminate one from the other.)