问题
ANSI-92 SQL mandates that comparisons with NULL evaluate to "falsy," eg:
SELECT * FROM table WHERE field = NULL
SELECT * FROM table WHERE field != NULL
Will both return no rows because NULL can't be compared like that. Instead, the predicates IS NULL and IS NOT NULL have to be used instead:
SELECT * FROM table WHERE field IS NULL
SELECT * FROM table WHERE field IS NOT NULL
Research has shown me that Oracle 1, PostgreSQL, MySQL and SQLite all support the ANSI syntax. Add to that list DB2 and Firebird.
Aside from SQL Server with ANSI_NULLS turned off, what other RDBMS support the non-ANSI syntax?
1 The whole empty string = NULL mess notwithstanding.
回答1:
Here is a nice comparison of null handling in SQLite, PostgreSQL, Oracle, Informix, DB2, MS-SQL, OCELOT, MySQL 3.23.41, MySQL 4.0.16, Firebird, SQL Anywhere, and Borland Interbase
回答2:
For what it's worth, comparing something to NULL is not strictly false, it's unknown. Furthermore, NOT unknown is still unknown.
ANSI SQL-99 defines a predicate IS [NOT] DISTINCT FROM. This allows you to mix nulls and non-null values in comarisons, and always get a true or false. Null compared to null in this way is true, otherwise any non-null compared to null is false. So negation works as you probably expect.
PostgreSQL, IBM DB2, and Firebird do support IS [NOT] DISTINCT FROM.
MySQL has a similar null-safe comparison operator <=> that returns true if the operands are the same and false if they're different.
Oracle has the hardest path. You have to get creative with use of NVL() or boolean expressions:
WHERE a = b OR (a IS NULL AND b IS NULL)
Yuck.
来源：https://stackoverflow.com/questions/7367327/comparisons-with-nulls-in-sql