3.8 IS NULL 运算符

在本教程中，您将学习如何使用 MySQL IS NULL运算符来测试值是否为一个NULL值。

MySQL IS NULL 操作符简介

要测试值是否为NULL值，需要使用IS NULL运算符。以下显示IS NULL运算符的语法：

value IS NULL

如果值为NULL，该表达式将返回true。否则返回false。

请注意，MySQL 没有内置的BOOLEAN类型。它使用TINYINT(1)来表示BOOLEAN值，即1表示true，0表示false。

因为IS NULL是一个比较运算符，所以您可以在任何使用运算符的地方使用它，例如在 SELECT 或 WHERE 子句中。如下面的例子：

SELECT 1 IS NULL,  # -- 0
       0 IS NULL,  # -- 0
       NULL IS NULL; # -- 1;

要检查值是否不为NULL，请使用IS NOT NULL运算符，如下所示：

value IS NOT NULL

如果该值不为NULL，则此表达式返回true(也就是1)。否则返回false(也就是0)。请考虑以下示例：

SELECT 1 IS NOT NULL, #-- 1
       0 IS NOT NULL, #-- 1
       NULL IS NOT NULL; #-- 0;

上面查询语句，执行后得到以下结果 -

mysql> SELECT 1 IS NOT NULL, #-- 1
       0 IS NOT NULL, #-- 1
       NULL IS NOT NULL; #-- 0
+---------------+---------------+------------------+
| 1 IS NOT NULL | 0 IS NOT NULL | NULL IS NOT NULL |
+---------------+---------------+------------------+
|             1 |             1 |                0 |
+---------------+---------------+------------------+
1 row in set

MySQL IS NULL 示例

我们将使用示例数据库(yiibaidb)中的customers表进行演示，customers表的结构如下所示 -

mysql> desc customers;
+------------------------+---------------+------+-----+---------+-------+
| Field                  | Type          | Null | Key | Default | Extra |
+------------------------+---------------+------+-----+---------+-------+
| customerNumber         | int(11)       | NO   | PRI | NULL    |       |
| customerName           | varchar(50)   | NO   |     | NULL    |       |
| contactLastName        | varchar(50)   | NO   |     | NULL    |       |
| contactFirstName       | varchar(50)   | NO   |     | NULL    |       |
| phone                  | varchar(50)   | NO   |     | NULL    |       |
| addressLine1           | varchar(50)   | NO   |     | NULL    |       |
| addressLine2           | varchar(50)   | YES  |     | NULL    |       |
| city                   | varchar(50)   | NO   |     | NULL    |       |
| state                  | varchar(50)   | YES  |     | NULL    |       |
| postalCode             | varchar(15)   | YES  |     | NULL    |       |
| country                | varchar(50)   | NO   |     | NULL    |       |
| salesRepEmployeeNumber | int(11)       | YES  | MUL | NULL    |       |
| creditLimit            | decimal(10,2) | YES  |     | NULL    |       |
+------------------------+---------------+------+-----+---------+-------+
13 rows in set

要查询没有销售代表的客户，请使用IS NULL运算符，如下所示：

SELECT 
    customerName, 
    country, 
    salesrepemployeenumber
FROM
    customers
WHERE
    salesrepemployeenumber IS NULL
ORDER BY customerName;

执行上面查询，得到以下结果 -

mysql> SELECT 
    customerName, 
    country, 
    salesrepemployeenumber
FROM
    customers
WHERE
    salesrepemployeenumber IS NULL
ORDER BY customerName;
+--------------------------------+--------------+------------------------+
| customerName                   | country      | salesrepemployeenumber |
+--------------------------------+--------------+------------------------+
| ANG Resellers                  | Spain        | NULL                   |
| Anton Designs, Ltd.            | Spain        | NULL                   |
| Asian Shopping Network, Co     | Singapore    | NULL                   |
| Asian Treasures, Inc.          | Ireland      | NULL                   |
| BG&E Collectables              | Switzerland  | NULL                   |
| Cramer Spezialitten, Ltd       | Germany      | NULL                   |
| Der Hund Imports               | Germany      | NULL                   |
| Schuyler Imports               | Netherlands  | NULL                   |
| Stuttgart Collectable Exchange | Germany      | NULL                   |
| Warburg Exchange               | Germany      | NULL                   |
... ...
+--------------------------------+--------------+------------------------+
22 rows in set

要查询有销售代表的客户，请使用IS NOT NULL运算符，如下查询语句 -

SELECT 
    customerName, 
    country, 
    salesrepemployeenumber
FROM
    customers
WHERE
    salesrepemployeenumber IS NOT NULL
ORDER BY customerName;

执行上面查询，得到以下结果 -

mysql> SELECT 
    customerName, 
    country, 
    salesrepemployeenumber
FROM
    customers
WHERE
    salesrepemployeenumber IS NOT NULL
ORDER BY customerName;
+------------------------------------+-------------+------------------------+
| customerName                       | country     | salesrepemployeenumber |
+------------------------------------+-------------+------------------------+
| Alpha Cognac                       | France      |                   1370 |
| American Souvenirs Inc             | USA         |                   1286 |
| Amica Models & Co.                 | Italy       |                   1401 |
| Anna's Decorations, Ltd            | Australia   |                   1611 |
| Atelier graphique                  | France      |                   1370 |
| Australian Collectables, Ltd       | Australia   |                   1611 |
| Australian Collectors, Co.         | Australia   |                   1611 |
| Australian Gift Network, Co        | Australia   |                   1611 |
| Auto Associs & Cie.                | France      |                   1370 |
| Auto Canal+ Petit                  | France      |                   1337 |
| Auto-Moto Classics Inc.            | USA         |                   1216 |
| AV Stores, Co.                     | UK          |                   1501 |
| Baane Mini Imports                 | Norway      |                   1504 |
| Bavarian Collectables Imports, Co. | Germany     |                   1504 |
... ...
+------------------------------------+-------------+------------------------+
100 rows in set

MySQL IS NULL 的专用功能

为了兼容 ODBC 程序，MySQL 支持IS NULL运算符的一些专门功能。

(1). 如果具有NOT NULL约束的DATE或DATETIME列包含特殊日期’0000-00-00‘，则可以使用IS NULL运算符来查找这些行。如下示例 -

CREATE TABLE IF NOT EXISTS projects (
    id INT AUTO_INCREMENT,
    title VARCHAR(255),
    begin_date DATE NOT NULL,
    complete_date DATE NOT NULL,
    PRIMARY KEY(id)
);

INSERT INTO projects(title,begin_date, complete_date)
VALUES('New CRM','2020-01-01','0000-00-00'),
      ('ERP Future','2020-01-01','0000-00-00'),
      ('VR','2020-01-01','2030-01-01');


SELECT 
    *
FROM
    projects
WHERE
    complete_date IS NULL;

在这个例子中，创建了一个 projects 新表，并将一些数据插入到表中。最后一个查询使用IS NULL来获取complete_date列中的值为“0000-00-00”的行。

(2). 如果变量@@sql_auto_is_null设置为1，则可以使用IS NULL运算符在执行INSERT语句后获取生成列的值。请注意，默认情况下，变量@@sql_auto_is_null为0。请参见以下示例。

首先，将变量@@sql_auto_is_null设置为1。

SET @@sql_auto_is_null = 1;

第二步，在projects表中插入一个新行：

INSERT INTO projects(title,begin_date, complete_date)
VALUES('MRP III','2010-01-01','2020-12-31');

第三步，使用IS NULL运算符来获取id列的生成值：

SELECT 
    id
FROM
    projects
WHERE
    id IS NULL;

MySQL IS NULL优化

MySQL 对于IS NULL运算符执行相同的优化方式与等于(=)运算符相同。

例如，MySQL 在使用IS NULL运算符搜索NULL时使用索引，如以下查询所示：

SELECT 
    customerNumber, 
    salesRepEmployeeNumber
FROM
    customers
WHERE
    salesRepEmployeeNumber IS NULL;

查看EXPLAIN查询过程：

EXPLAIN SELECT 
    customerNumber, 
    salesRepEmployeeNumber
FROM
    customers
WHERE
    salesRepEmployeeNumber IS NULL;

执行上面查询语句，输出以下结果 -

mysql> EXPLAIN SELECT 
    customerNumber, 
    salesRepEmployeeNumber
FROM
    customers
WHERE
    salesRepEmployeeNumber IS NULL;
+----+-------------+-----------+------------+------+------------------------+------------------------+---------+-------+------+----------+--------------------------+
| id | select_type | table     | partitions | type | possible_keys          | key                    | key_len | ref   | rows | filtered | Extra                    |
+----+-------------+-----------+------------+------+------------------------+------------------------+---------+-------+------+----------+--------------------------+
|  1 | SIMPLE      | customers | NULL       | ref  | salesRepEmployeeNumber | salesRepEmployeeNumber | 5       | const |   22 |      100 | Using where; Using index |
+----+-------------+-----------+------------+------+------------------------+------------------------+---------+-------+------+----------+--------------------------+
1 row in set

MySQL 也可以优化组合col = value OR col IS NULL。请参阅以下示例：

EXPLAIN SELECT 
    customerNumber,
    salesRepEmployeeNumber
FROM
    customers
WHERE
    salesRepEmployeeNumber = 1370 OR
    salesRepEmployeeNumber IS NULL;

执行上面查询语句，得到以下结果 -

mysql> EXPLAIN SELECT 
    customerNumber,
    salesRepEmployeeNumber
FROM
    customers
WHERE
    salesRepEmployeeNumber = 1370 OR
    salesRepEmployeeNumber IS NULL;
+----+-------------+-----------+------------+-------------+------------------------+------------------------+---------+-------+------+----------+--------------------------+
| id | select_type | table     | partitions | type        | possible_keys          | key                    | key_len | ref   | rows | filtered | Extra                    |
+----+-------------+-----------+------------+-------------+------------------------+------------------------+---------+-------+------+----------+--------------------------+
|  1 | SIMPLE      | customers | NULL       | ref_or_null | salesRepEmployeeNumber | salesRepEmployeeNumber | 5       | const |   29 |      100 | Using where; Using index |
+----+-------------+-----------+------------+-------------+------------------------+------------------------+---------+-------+------+----------+--------------------------+
1 row in set

在这个例子中，当应用优化时，EXPLAIN会显示ref_or_null。

如果您有一个列的组合键，MySQL可以对任何关键部分执行优化。假设在表t1的列c1和c2上有一个索引，以下查询被优化：

SELECT 
    *
FROM
    t1
WHERE
    c1 IS NULL;

在本教程中，您已经学习了如何使用 MySQL IS NULL运算符来测试值是否为NULL。

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