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顯然，ISNULL()函數(shù)和IS NULL運(yùn)算符沒有任何區(qū)別，并且共享一些共同的行為。我們唯一能看到的區(qū)別是它們的語法。ISNULL()函數(shù)將表達(dá)式作為其參數(shù)，而IS NULL比較運(yùn)算符將表達(dá)式放在其左側(cè)。否則，如果表達(dá)式為NULL，則兩者都返回1，如果表達(dá)式不為NULL，則返回0。以下示例將演示上述概念?

mysql> Select 1 IS NULL;
+-----------+
| 1 IS NULL |
+-----------+
| 0         |
+-----------+
1 row in set (0.00 sec)

mysql> Select ISNULL(1);
+-----------+
| ISNULL(1) |
+-----------+
| 0         |
+-----------+
1 row in set (0.00 sec)

mysql> Select ISNULL(1/0);
+-------------+
| ISNULL(1/0) |
+-------------+
| 1           |
+-------------+
1 row in set (0.00 sec)

mysql> Select 1/0 IS NULL;
+-------------+
| 1/0 IS NULL |
+-------------+
| 1           |
+-------------+
1 row in set (0.00 sec)

mysql> Select * from Employee WHERE Salary IS NULL;
+----+-------+--------+
| ID | Name  | Salary |
+----+-------+--------+
| 7  | Aryan | NULL   |
| 8  | Vinay | NULL   |
+----+-------+--------+
2 rows in set (0.00 sec)

mysql> Select * from Employee WHERE ISNULL(Salary);
+----+-------+--------+
| ID | Name | Salary |
+----+-------+--------+
| 7  | Aryan | NULL   |
| 8  | Vinay | NULL   |
+----+-------+--------+
2 rows in set (0.00 sec)

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