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I've got a vague, possibly cargo-cult memory from years of working with SQL Server that when you've got a possibly-null column, it's not safe to write "WHERE" clause predicates like:
... WHERE the_column IS NULL OR the_column < 10 ...
It had something to do with the fact that SQL rules don't stipulate short-circuiting (and in fact that's kind-of a bad idea possibly for query optimization reasons), and thus the "<" comparison (or whatever) could be evaluated even if the column value is null. Now, exactly why that'd be a terrible thing, I don't know, but I recall being sternly warned by some documentation to always code that as a "CASE" clause:
... WHERE 1 = CASE WHEN the_column IS NULL THEN 1 WHEN the_column < 10 THEN 1 ELSE 0 END ...
(the goofy "1 = " part is because SQL Server doesn't/didn't have first-class booleans, or at least I thought it didn't.)
So my questions here are:
My grounding in SQL is pretty weak.
927
In SQL ternary logic, NULL equates to the value of unknown. The take-away: Any logical operation involving a NULL results in a value of unknown except for TRUE OR NULL and FALSE AND NULL.
The NULL is not zero or blank. It represents an unknown or inapplicable value. It can't be compared using AND / OR logical operators. The special operator 'IS' is used with the keyword 'NULL' to locate 'NULL' values.
A field with a NULL value is a field with no value. If a field in a table is optional, it is possible to insert a new record or update a record without adding a value to this field. Then, the field will be saved with a NULL value. Note: A NULL value is different from a zero value or a field that contains spaces.
You can use the IS NULL operator to test whether a given value expression is Null: . . .
Instead of
the_column IS NULL OR the_column < 10
I'd do
isnull(the_column,0) < 10
or for the first example
WHERE 1 = CASE WHEN isnull(the_column,0) < 10 THEN 1 ELSE 0 END ...
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