我有类似的疑问

/* Query */
global $wpdb;
$tablename = $wpdb->prefix . \'data\';
$sql = $wpdb->prepare(
    "
    UPDATE $tablename
    SET
        `date` = %s,
    WHERE
        id= %d
    ",
    $_POST[\'date\'] == \'\' ? "NULL": $_POST[\'date\'],
    $_POST[\'id\']
);
$wpdb->query($sql);

这将导致：

UPDATE `date` SET \'NULL\' WHERE `id` = $_POST[\'id\'] 

因此prepare（）将单引号添加到NULL，查询将字段设置为NULL string not NULL value。我唯一的修复方法是在prepare（）函数之外使用变量，如下所示：

/* Query */
global $wpdb;
$tablename = $wpdb->prefix . \'data\';

/* Here I declare the variabile outside of the prepare() */
$date = $output[\'date\'] == \'\' ? "NULL" : $_POST[\'date\'];
$sql = $wpdb->prepare(
    "
    UPDATE $tablename
    SET
        `date` = $date,
    WHERE
        id = %d
    ",
    $_POST[\'id\']
);
$wpdb->query($sql);

/* Query */
global $wpdb;
$tablename = $wpdb->prefix . \'data\';
$sql = $wpdb->prepare(
    "
    UPDATE $tablename
    SET
        `date` = %s,
    WHERE
        id= %d
    ",
    $_POST[\'date\'],
    $_POST[\'id\']
);
// SQL = UPDATE prefix_data SET `date` = \'\' WHERE id = 1


$sql = str_replace("\'\'",\'NULL\', $sql);
// SQL = UPDATE prefix_data SET `date` = NULL WHERE id = 1

$wpdb->query($sql);