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我的数据库中有下表：

CREATE TABLE Alita.Cache (
ID              INT             NOT NULL AUTO_INCREMENT,
Created         DATETIME        NOT NULL,
Modified        DATETIME        NOT NULL,
ProcessState    TINYINT         NOT NULL,
Link            VARCHAR(4000)   CHARSET utf8,
Content         VARCHAR(4000)   CHARSET utf8,

CONSTRAINT PK_CACHE_ID    PRIMARY KEY (ID));

要在此表中添加一行，我创建了以下存储过程：

DELIMITER //
DROP PROCEDURE IF EXISTS Alita.usp_AddCache //
CREATE PROCEDURE Alita.usp_AddCache (
    IN  _link    VARCHAR(4000) CHARSET utf8,
    IN  _content VARCHAR(4000) CHARSET utf8,
    OUT _linkId  INT
 )
PROC_START : BEGIN
START TRANSACTION;

SET @id := NULL;
SELECT @id := Id FROM Alita.Cache WHERE Link = _link;
SET _linkId := @id;

IF @id IS NULL THEN 
    INSERT INTO Alita.Cache VALUES(0, NOW(), NOW(), 0, _link, _content);
    SELECT @id := Id FROM Alita.Cache WHERE Link = _link;
    SET _linkId := @id;
    LEAVE PROC_START;
END IF;

UPDATE Alita.Cache 
SET Content = _content, 
    Modified = NOW(), 
    ProcessState = 0 
    WHERE Id = @id AND ProcessState = 2;

COMMIT;
END //
DELIMITER ;

现在我尝试从 MySql C++ 连接器调用这个存储过程。我将所有数据库交互代码封装在一个名为alita_db的类中。我使用了 MySQL 文档中的本教程。以下片段是连接的初始化：

alita::alita_db::alita_db(const alita::db_connection_info& db_connection_info)
{
    if(db_connection_info._host.empty())
        throw std::invalid_argument("Invalid Host.");

    if(db_connection_info._username.empty())
        throw std::invalid_argument("Invalid Username.");

    if(db_connection_info._password.empty())
        throw std::invalid_argument("Invalid Password.");

    if(db_connection_info._scheme.empty())
        throw std::invalid_argument("Invalid Scheme.");

    this->_log = db_connection_info._log;
    this->driver = get_driver_instance();
    this->connection = this->driver->connect(
        db_connection_info._host,
        db_connection_info._username,
        db_connection_info._password);

    this->connection->setSchema(db_connection_info._scheme);  
    this->connection->setAutoCommit(false);
}

初始化后，我尝试执行调用usp_AddCache。以下是使用 MySql C++ 连接器 API 的调用片段。

    std::unique_ptr<sql::Statement> stmt;
    std::unique_ptr<sql::PreparedStatement> pstmt;
    std::unique_ptr<sql::ResultSet> res;

    stmt.reset(this->connection->createStatement());
    pstmt.reset(this->connection->prepareStatement("CALL usp_AddCache(?, ?, @_linkId)"));

    pstmt->setString(1, std::wstring_convert<std::codecvt_utf8<wchar_t>, wchar_t>().to_bytes(link));
    pstmt->setString(2, std::wstring_convert<std::codecvt_utf8<wchar_t>, wchar_t>().to_bytes(content));

    pstmt->execute();

代码成功执行，没有任何异常，并且自动递增 ID 计数器已更改。然而，当我使用 mysql CLI 查询数据时，我得到一个空集。我试图理解为什么插入实际上失败了。我的问题是如何解决这个问题。

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