Question

如果数据库中不存在作者，我试图让这个函数只插入作者。

如果作者已经在'authors'表中（意思是相同的名字和相同的姓），那么我希望这个函数不插入作者但我还是希望它返回作者的ID而我不确定怎么做。

这是我的insertAuthor函数，即使他已经存在，也会插入作者：

public static int insertAuthor(Connection conn, Author author) throws SQLException
{
    ResultSet keys = null;
    int authorId;

    // Insert the new Authors into the authors table
    String sqlInsertAuthor = "INSERT into authors"
            + "(author_firstname, author_lastname)"
            + " VALUES (?, ?)";

    try (PreparedStatement statement = conn.prepareStatement(sqlInsertAuthor, Statement.RETURN_GENERATED_KEYS);)
    {
        statement.setString(1, author.getFirstName());
        statement.setString(2, author.getLastName());

        // Execute and return number of rows affected
        int affectedRows = statement.executeUpdate();

        // The number of affected rows should be equal to 1
        if (affectedRows == 1)
        {
            keys = statement.getGeneratedKeys();
            keys.next();
            authorId = keys.getInt(1);
        }
        else
        {
            return 0;
        }
    }
    catch (SQLException e)
    {
        System.err.println("ERROR INSERTING AUTHOR: " + e.getMessage());
        return 0;
    }

    return authorId;
}

Answer 1

嗯，有几种方法可以做到这一点。
您可以使用hibernate和它的二级缓存来加载所有作者并检查名字和姓氏。
您可以将lastname设置为唯一 - 如果lastname已存在，则会引发错误。
您可以手动执行 - 只需检查名字和姓氏是否已存在（选择first name和lastname的计数）。如果0添加新行。等等。

如果你想要id，那么最后一个解决方案可能是最好的：而不是计数你可以选择in - 如果没有，则会有null。

Answer 2

好的，当我尝试kamirru的选择计数时，我设法得到了我想要的东西。

以下是工作代码：

public static int insertAuthor(Connection conn, Author author) throws SQLException
{
    ResultSet keys = null;
    int authorId;

    // Insert the new Authors into the authors table
    String sqlInsertAuthor = "INSERT into authors"
            + "(author_firstname, author_lastname)"
            + " VALUES (?, ?)";

    try (PreparedStatement statement = conn.prepareStatement(sqlInsertAuthor, Statement.RETURN_GENERATED_KEYS);)
    {
        // Check if author already exists, if yes return his id
        authorId = authorExists(conn, author.getFirstName(), author.getLastName());

        // If 0 is returned then the author does not exist so insert the author
        if (authorId == 0)
        {
            statement.setString(1, author.getFirstName());
            statement.setString(2, author.getLastName());

            // Execute and return number of rows affected
            int affectedRows = statement.executeUpdate();

            // The number of affected rows should be equal to 1
            if (affectedRows == 1)
            {
                keys = statement.getGeneratedKeys();
                keys.next();
                authorId = keys.getInt(1);
            }
            else
            {
                return 0;
            }
        }
    }
    catch (SQLException e)
    {
        System.err.println("ERROR INSERTING AUTHOR: " + e.getMessage());
        return 0;
    }

    return authorId;
}

public static int authorExists(Connection conn, String firstName, String lastName) throws SQLException
{
    int authorId = 0;

    // Select the author to check if he exists
    String sqlCountAuthor = "SELECT author_id"
            + " FROM authors"
            + " WHERE author_firstname = ? AND author_lastname = ?";
    ResultSet rs = null;

    try (PreparedStatement statement = conn.prepareStatement(sqlCountAuthor);)
    {
        statement.setString(1, firstName);
        statement.setString(2, lastName);

        rs = statement.executeQuery();

        // If a result is returned then the author already exists so take his id
        if (rs.next())
        {
            authorId = rs.getInt("author_id");
        }
    }
    catch (SQLException e)
    {
        System.err.println("ERROR SELECTING AUTHOR: " + e.getMessage());
    }
    finally
    {
        if (rs != null)
        {
            rs.close();
        }
    }

    return authorId;
}

如果作者不存在，则将作者插入表格，但始终返回其ID

2 个答案: