问题
On the Wikipedia page for SQL there are some truth tables about boolean logic in SQL. [1] The Wikipedia page seems to source the SQL:2003 standard.
The truth table for the equals operator (=) is different from the IS operator from the SQL:2003 draft.
Also, the Wikipedia article notes that "IS NULL" (<null predicate>) is a special case.
In the SQL:2003 it seems that there is an "IS" opeartor which is a regular operator like AND, NOT and OR. However, the <null predicate> is still there.
Why is the <null predicate> there when the IS is a regular boolean operator? Is it to make sure you can use the "IS NULL" construct with non-boolean values without type coersion? Is it discouraged to use "=NULL"?
Does the SQL:2011 standard work differently?
[1]: Wikipedia on SQL
[2]: SQL:2011 draft PDF page 335
[3]: SQL:2003 draft PDF page 397
回答1:
That's a new one for me.
If I read that correctly the <boolean value expression> grammar defines three predicates solely for use with the boolean datatype IS TRUE, IS FALSE, IS UNKNOWN.
These differ from their equality counterparts in that they only evaluate to True or False. Never to Unknown. i.e. UNKNOWN = TRUE would evaluate to UNKNOWN but UNKNOWN IS TRUE evaluates to False.
The full truth tables for IS and = are below.
+---------+-------+-------+---------+
| IS | TRUE | FALSE | UNKNOWN |
+---------+-------+-------+---------+
| TRUE | TRUE | FALSE | FALSE |
| FALSE | FALSE | TRUE | FALSE |
| UNKNOWN | FALSE | FALSE | TRUE |
+---------+-------+-------+---------+
As opposed to
+---------+---------+---------+---------+
| = | TRUE | FALSE | UNKNOWN |
+---------+---------+---------+---------+
| TRUE | TRUE | FALSE | UNKNOWN |
| FALSE | FALSE | TRUE | UNKNOWN |
| UNKNOWN | UNKNOWN | UNKNOWN | UNKNOWN |
+---------+---------+---------+---------+
回答2:
As the above poster said, null = null is not correct. It will return NULL(false)
For null comparison you must use IS NULL or IS NOT NULL.
来源：https://stackoverflow.com/questions/9822154/standard-sql-boolean-operator-is-vs-equals-operator