Question

I am attempting to perform an inner join on two tables so that I can round a value by the precision associated with that value’s pid in the other table. However, it is returning NULL for pid 3. I believe it has to do with the NULL precision for pid 2, because when I set it’s precision to a non-NULL value, I get the correct value for pid 3 in the query below.

How can I restructure this query so that it will always return the correct value even if another pid contains a NULL precision?

select pid_values.pid, pid_values.value, round(value, pids.precision) 
from pid_values inner join pids on pid_values.pid = pids.pid;

+------+--------+------------------------------+
| pid  | value  | round(value, pids.precision) |
+------+--------+------------------------------+
|    1 | 10.123 |                           10 |
|    2 |    5.5 |                            6 |
|    3 | 3.1234 |                         NULL |
+------+--------+------------------------------+

Data in both tables:

select * from pids;
+------+-----------+
| pid  | precision |
+------+-----------+
|    1 |         0 |
|    2 |      NULL |
|    3 |         1 |
+------+-----------+

select * from pid_values
+----+------+--------+
| id | pid  | value  |
+----+------+--------+
|  1 |    1 | 10.123 |
|  2 |    2 |    5.5 |
|  3 |    3 | 3.1234 |
+----+------+--------+

Answer 1

Try this query:

SELECT 
  pid_values.pid, 
  pid_values.value, 
  round(value, IF(pids.precision IS NULL, 0, pids.precision)) 
FROM pid_values INNER JOIN pids ON pid_values.pid = pids.pid;