Mysqli 简明教程
MySQLi - Handling NULL Values
我们已经看到了 SQL SELECT 命令以及 WHERE 子句，用于从 MySQL 表中获取数据，但是当我们尝试给出一个条件时，该条件将字段或列值与 NULL 进行比较，它不能正常工作。
We have seen the SQL SELECT command along with the WHERE clause to fetch data from a MySQL table, but when we try to give a condition, which compares the field or the column value to NULL, it does not work properly.
为了处理这种情况，MySQL 提供了三个运算符 -
To handle such a situation, MySQL provides three operators −
-
IS NULL − This operator returns true, if the column value is NULL.
-
IS NOT NULL − This operator returns true, if the column value is not NULL.
-
<⇒ − This operator compares values, which (unlike the = operator) is true even for two NULL values.
涉及 NULL 的条件是特殊的。不能使用 = NULL 或 != NULL 来查找列中的 NULL 值。这种比较始终失败，因为无法判断它们是真还是假。有时，甚至 NULL = NULL 会失败。
The conditions involving NULL are special. You cannot use = NULL or != NULL to look for NULL values in columns. Such comparisons always fail because it is impossible to tell whether they are true or not. Sometimes, even NULL = NULL fails.
要查找存在或不存在 NULL 的列，请使用 IS NULL 或 IS NOT NULL 。
To look for columns that are or are not NULL, use IS NULL or IS NOT NULL.
Using NULL values at the Command Prompt
假设 TUTORIALS 数据库中有一个名为 tcount_tbl 的表，它包含两列，即 tutorial_author 和 tutorial_count ，其中 tutorial_count 为 NULL 表示该值未知。
Assume that there is a table called tcount_tbl in the TUTORIALS database and it contains two columns namely tutorial_author and tutorial_count, where a NULL tutorial_count indicates that the value is unknown.
Example
尝试以下示例：
Try the following examples −
root@host# mysql -u root -p password;
Enter password:*******
mysql> use TUTORIALS;
Database changed
mysql> create table tcount_tbl
→ (
→ tutorial_author varchar(40) NOT NULL,
→ tutorial_count INT
→ );
Query OK, 0 rows affected (0.05 sec)
mysql> INSERT INTO tcount_tbl
→ (tutorial_author, tutorial_count) values ('mahran', 20);
mysql> INSERT INTO tcount_tbl
→ (tutorial_author, tutorial_count) values ('mahnaz', NULL);
mysql> INSERT INTO tcount_tbl
→ (tutorial_author, tutorial_count) values ('Jen', NULL);
mysql> INSERT INTO tcount_tbl
→ (tutorial_author, tutorial_count) values ('Gill', 20);
mysql> SELECT * from tcount_tbl;
+-----------------+----------------+
| tutorial_author | tutorial_count |
+-----------------+----------------+
| mahran | 20 |
| mahnaz | NULL |
| Jen | NULL |
| Gill | 20 |
+-----------------+----------------+
4 rows in set (0.00 sec)
mysql>可以看到， = 和 != 不适用于 NULL 值，如下所示：
You can see that = and != do not work with NULL values as follows −
mysql> SELECT * FROM tcount_tbl WHERE tutorial_count = NULL;
Empty set (0.00 sec)
mysql> SELECT * FROM tcount_tbl WHERE tutorial_count != NULL;
Empty set (0.01 sec)要查找 tutorial_count 列存在或不存在的记录，应按以下程序中所示编写查询。
To find the records where the tutorial_count column is or is not NULL, the queries should be written as shown in the following program.
mysql> SELECT * FROM tcount_tbl
→ WHERE tutorial_count IS NULL;
+-----------------+----------------+
| tutorial_author | tutorial_count |
+-----------------+----------------+
| mahnaz | NULL |
| Jen | NULL |
+-----------------+----------------+
2 rows in set (0.00 sec)
mysql> SELECT * from tcount_tbl
→ WHERE tutorial_count IS NOT NULL;
+-----------------+----------------+
| tutorial_author | tutorial_count |
+-----------------+----------------+
| mahran | 20 |
| Gill | 20 |
+-----------------+----------------+
2 rows in set (0.00 sec)Handling NULL Values in a PHP Script
可以使用 if…else 条件根据 NULL 值准备查询。
You can use the if…else condition to prepare a query based on the NULL value.
以下示例从外部获取 tutorial_count，然后将其与表中可用值进行比较。
The following example takes the tutorial_count from outside and then compares it with the value available in the table.
Example
将以下示例复制粘贴为 mysql_example.php：
Copy and paste the following example as mysql_example.php −
<html>
<head>
<title>Handling NULL</title>
</head>
<body>
<?php
$dbhost = 'localhost';
$dbuser = 'root';
$dbpass = 'root@123';
$dbname = 'TUTORIALS';
$mysqli = new mysqli($dbhost, $dbuser, $dbpass, $dbname);
$tutorial_count = null;
if($mysqli→connect_errno ) {
printf("Connect failed: %s<br />", $mysqli→connect_error);
exit();
}
printf('Connected successfully.<br />');
if( isset($tutorial_count )) {
$sql = 'SELECT tutorial_author, tutorial_count
FROM tcount_tbl
WHERE tutorial_count = ' + $tutorial_count;
} else {
$sql = 'SELECT tutorial_author, tutorial_count
FROM tcount_tbl
WHERE tutorial_count IS NULL';
}
$result = $mysqli→query($sql);
if ($result→num_rows > 0) {
while($row = $result→fetch_assoc()) {
printf("Author: %s, Count: %d <br />",
$row["tutorial_author"],
$row["tutorial_count"]);
}
} else {
printf('No record found.<br />');
}
$mysqli→close();
?>
</body>
</html>