Re: Differentiate Between Zero-Length String and NULLColumn Values

Peter Eisentraut wrote:
> D'Arcy J.M. Cain wrote:
>
>> SELECT * FROM table WHERE column IS NULL;
>> SELECT * FROM table WHERE column = NULL;
>>
>> The latter violates the SQL spec and is not allowed by PostgreSQL
>> without setting a special flag.
>>
>
> It doesn't violate any spec and it's certainly allowed by PostgreSQL
> without any flags. It's just that the result is not what some people
> expect.
>
>

"= NULL" violates the SQL-92 Specification. Relevant section posted below.
Additionally *none of the example code* in the SQL-92 specification
document uses the expression "WHERE #value# = NULL"
*All* the example code in the specification use the expression as:
"WHERE #value# IS NULL"

8.6 <null predicate>

Function

Specify a test for a null value.

Format

<null predicate> ::= <row value constructor> IS [ NOT ] NULL

Syntax Rules

None.

Access Rules

None.

General Rules

1) Let R be the value of the <row value constructor>.

2) If all the values in R are the null value, then "R IS NULL" is
true; otherwise, it is false.

3) If none of the values in R are the null value, then "R IS NOT
NULL" is true; otherwise, it is false.

Note: For all R, "R IS NOT NULL" has the same result as "NOT
R IS NULL" if and only if R is of degree 1. Table 12, "<null
predicate> semantics", specifies this behavior.

________________Table_12-<null_predicate>_semantics________________

R IS R IS NOT NOT R IS NOT R IS NOT
_Expression_______NULL____NULL__________NULL__________NULL_________

| degree 1: null | true | false | false | true |
| | | | | |
| degree 1: not | false | true | true | false |
null

| degree > 1: | true | false | false | true |
| all null | | | | |
| | | | | |
| degree > 1: | false | false | true | true |
| some null | | | | |
| | | | | |
| degree > 1: | false | true | true | false |
|_none_null______|_______|_____________|____________|______________|
| | | | | |
|Leveling Rules | | | | |
| | | | | |
218 Database Language SQL