Le 29/03/2013 21:57, Girvin R. Herr a écrit :
Hi Girvin,
> Why not use NULLs? I use them all the time when a field is unknown.
> The only places I do not allow NULLs are the primary and foreign keys
> and maybe some fields that are in calculated field formulas, such as:
> <price> * <quantity>. Hope this helps.
That is exactly the problem. Using NULL as an allowable value in
concatenated or function associated string manipulation will, depending
on the db engine, potentially lead to a NULL result being returned where
one of the values to be concatenated is NULL, see the example below for
mysql :
mysql> select * from devices;
+----+------+-------+----------------+--------------------------------------+------------+-----------+
| id | type | label | lastmountpoint | uuid
| servername | sharename |
+----+------+-------+----------------+--------------------------------------+------------+-----------+
| 1 | uuid | NULL | /home |
97be65ae-c0b9-4f3d-9edc-2c65afe15311 | NULL | NULL |
| 2 | uuid | NULL | / |
0e98860b-bd1e-4e78-ab42-2d2adbb18eed | NULL | NULL |
+----+------+-------+----------------+--------------------------------------+------------+-----------+
2 rows in set (0.02 sec)
mysql> select concat(type,label,uuid) from devices;
+-------------------------+
| concat(type,label,uuid) |
+-------------------------+
| NULL |
| NULL |
+-------------------------+
2 rows in set (0.00 sec)
If Dan's aim is to concatenate the components of the name in a query,
then he'll be stuffed if any of those fields have NULL values.
Alex
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