mysqli唯一索引异常

$sql = "SELECT * FROM table_name WHERE column_name = ?";
$stmt = $mysqli->prepare($sql);
$stmt->bind_param("s", $value);
$stmt->execute();
$result = $stmt->get_result();
if ($result->num_rows > 0) {
    echo "数据已存在";
} else {
    // 插入数据
}

SHOW INDEX FROM table_name WHERE Key_name = 'index_name';

ALTER TABLE table_name DROP INDEX index_name;
ALTER TABLE table_name ADD UNIQUE INDEX index_name (column_name);

ALTER TABLE table_name MODIFY column_name datatype;

// 连接数据库
$mysqli = new mysqli("localhost", "username", "password", "database");

// 检查数据是否存在
$email = "example@example.com";
$sql = "SELECT * FROM users WHERE email = ?";
$stmt = $mysqli->prepare($sql);
$stmt->bind_param("s", $email);
$stmt->execute();
$result = $stmt->get_result();
if ($result->num_rows > 0) {
    echo "电子邮件已存在";
} else {
    // 插入数据
    $sql = "INSERT INTO users (email, name) VALUES (?, ?)";
    $stmt = $mysqli->prepare($sql);
    $stmt->bind_param("ss", $email, "John Doe");
    $stmt->execute();
    echo "数据插入成功";
}