Gen and null columns

A recent posting on the Duick forum regarding NULL column support lead to a discussion on the Gen qualifier IS EQUIVALENT TO and a potential misinterpretation of the way that this works.

As a bid of background information, it is important to understand how a nullable column containing a NULL value behaves. Consider a table with a nullable column and these rows:

Id opt_column
1 ' ' (column has a value of spaces)
2 NULL(column is NULL
3 'X' (column has a value of X)

If you want a value of NULL to be equivalent to SPACES, and you want to read rows that have a space or null in the opt_column, then if your SQL was:

SELECT * FROM table WHERE opt_column = ' ';

would return just row 1 but

SELECT * FROM table WHERE opt_column = ' ' OR opt_column IS NULL;

would return rows 1 and 2

If you want to read rows that do not have ‘X’:

SELECT * FROM table WHERE opt_column != 'X';

would return just row 1 but

SELECT * FROM table WHERE opt_column != 'X' OR opt_column IS NULL;

would return rows 1 and 2

Once you understand the need for the IS NULL or IS NOT NULL qualifier in the SQL, you can write the READ qualifiers in the action diagram code.

The confusion arises over the use of the IS EQUIVALENT TO clause since it is likely that this does not work the way you expect!

For example, if an optional column has no value, then I think of SPACES & NULL as the same, so you would code:

READ table WHERE opt_column = SPACES OR opt_column IS NULL

However the statement READ table WHERE opt_column IS EQUIVALENT TO SPACES

gives the following SQL which is not the same:

SELECT opt_column FROM table
WHERE (opt_column = ' ' AND opt_column IS NOT NULL)

This means that if the column is NULL it will not return a row, which is the opposite of what I think you want.

IS EQUIVALENT TO is OK. For example, if you want a row with a specific value:

READ table WHERE opt_column = ‘X’ AND opt_column IS NOT NULL

Is the same as READ table WHERE opt_column IS EQUIVALENT TO ‘X’

which gives the following SQL:

SELECT opt_column FROM table
WHERE (opt_column = 'X' AND opt_column IS NOT NULL)

IS NOT EQUIVALENT also gives the desired result for a specific value but not for SPACES:

READ table WHERE opt_column NOT = ‘X’ OR opt_column IS NULL

and READ table WHERE opt_column IS NOT EQUIVALENT TO ‘X’ gives:

SELECT opt_column FROM table
WHERE (opt_column <> 'X' OR opt_column IS NULL)

But if you want a row where the column is not spaces, you would code:

READ table WHERE opt_column NOT = SPACES AND opt_column IS NOT NULL

but READ table WHERE opt_column IS NOT EQUIVALENT TO SPACES gives:

SELECT opt_column FROM table
WHERE (opt_column <> ' ' OR opt_column IS NULL)

In summary, it is best not to use EQUIVALENT with SPACES unless you want the behaviour that the generated code gives you. You would also need to be careful with a view that might have a value of spaces.