Question

I’m wondering how makumba is handling uniqueness validation rules between multiple fields, when there are null fields involved.

For instance:
a = not null not empty char…
b = ptr …
unique(a, b)

What’ll happen in the cases where ‘b’ is null. Will this check be done with ‘+null’ or will it be bypassed?

Answer 1

There are two different cases

(1) If the validation rule covers only fields from one MDD, then this will be translated into a unique constraint on the database level, and how that is handled there might be depending on the DB engine used.

In mysql, null values are allowed to be repeated (http://dev.mysql.com/doc/refman/5.1/en/create-table.html)

A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. For all engines, a UNIQUE index permits multiple NULL values for columns that can contain NULL.

I.e. you can actually have identical records!

You can verify that:

mysql> CREATE TABLE example (data1 VARCHAR(100), data2 varchar(100), data3 varchar(100));
mysql> alter table example add unique index(data1, data2, data3);

mysql> insert into example values ("a", "a", "a");
Query OK, 1 row affected (0.00 sec)
mysql> insert into example values ("a", "a", "a");
ERROR 1062 (23000): Duplicate entry 'a-a-a' for key 'data1'
mysql> insert into example values ("a", "a", null);
Query OK, 1 row affected (0.00 sec)
mysql> insert into example values ("a", "a", null);
Query OK, 1 row affected (0.00 sec)
mysql> insert into example values ("a", "a", null);
Query OK, 1 row affected (0.00 sec)
mysql> insert into example values ("a", "a", null);
Query OK, 1 row affected (0.00 sec)

mysql> select * from example;
+-------+-------+-------+
| data1 | data2 | data3 |
+-------+-------+-------+
| a     | a     | NULL  |
| a     | a     | NULL  |
| a     | a     | NULL  |
| a     | a     | NULL  |
| a     | a     | a     |
+-------+-------+-------+

So if applicable, you might want to add a not-null statement to the fields in question.

(2) If the rule covers also fields from other MDDs (e.g. via a ptr field), then this will be checked via a query – if there already exists a record that matches the query, the uniqueness check will complain.
In this case null should be treated as any other value would be; i.e. you should not be able to have two records that have two identical records