问题是，您想获取主题，而不是学生。因此，我反转了FROM和LEFT JOIN。因此，当您想要主题列表时，您将从SELECT …
FROM主题开始。然后，如果您需要每个主题的其他详细信息（例如用户名等），请应用LEFT
JOIN，这意味着：将所有必需的详细信息（用户名等）加入LEFT表的每个记录（例如主表）中您的情况是表格“主题”）。

祝好运！

<?php

try {
    $dbAdapter = new DbAdapter();

    $connection = $dbAdapter->connect();

    /*
     * I renamed user id variable (from $userId to $userid1) in order to show you that you can 
     * provide more users if you wish. Then you just have to extend 
     * the WHERE clause in the sql statement and the bindings array.
     */
    $userid1 = 1;

    /*
     * The sql statement - it will be prepared.
     * 
     * ======================================================
     * I'm not sure about the following fields - because you
     * didn't provide proper selection criteria for them:
     * 
     * 1) "subjectsid":  "1", - first subject id for the student in this case the one for phy
     * 2) "subjectname": "bio", - current subject name 
     * ======================================================
     */
    $sql = 'SELECT 
                subjects.userid,
                users.name AS username,
                (
                    SELECT id 
                    FROM tbsubjects 
                    WHERE userid = subjects.userid 
                    ORDER BY id ASC 
                    LIMIT 1
                ) AS subjectsid,
                (
                    SELECT name 
                    FROM tbsubjects 
                    WHERE 
                        userid = subjects.userid 
                        ORDER BY time DESC
                        LIMIT 1
                ) AS subjectname,
                (
                    SELECT IFNULL(SUM(points), 0) 
                    FROM tbsubjects 
                    WHERE 
                        userid = subjects.userid 
                        AND month = DATE_FORMAT(NOW(), "%c")
                ) AS activepts,
                IFNULL(SUM(subjects.points), 0) AS totalpts,
                (
                    SELECT IFNULL(SUM(points), 0) 
                    FROM tbsubjects 
                    WHERE 
                        userid = subjects.userid 
                        AND semester = 1
                ) AS sem1,
                (
                    SELECT IFNULL(SUM(points), 0) 
                    FROM tbsubjects 
                    WHERE 
                        userid = subjects.userid 
                        AND semester = 2
                ) AS sem2,
                (
                    SELECT IFNULL(SUM(points), 0) 
                    FROM tbsubjects 
                    WHERE 
                        userid = subjects.userid 
                        AND semester = 3
                ) AS sem3 
            FROM 
                tbsubjects AS subjects 
                LEFT JOIN tbusers AS users ON users.id = subjects.userid 
            WHERE subjects.userid = :userid1 
            GROUP BY subjects.userid 
            ORDER BY subjects.time DESC';

    /*
     * The input parameters list for the prepared sql statement.
     */
    $bindings = array(
        ':userid1' => $userid1,
    );

    /*
     * Prepare and validate the sql statement.
     * 
     * --------------------------------------------------------------------------------
     * If the database server cannot successfully prepare the statement, PDO::prepare() 
     * returns FALSE or emits PDOException (depending on error handling settings).
     * --------------------------------------------------------------------------------
     */
    $statement = $connection->prepare($sql);

    if (!$statement) {
        throw new UnexpectedValueException('The sql statement could not be prepared!');
    }

    /*
     * Bind the input parameters to the prepared statement.
     * 
     * -----------------------------------------------------------------------------------
     * Unlike PDOStatement::bindValue(), when using PDOStatement::bindParam() the variable 
     * is bound as a reference and will only be evaluated at the time that 
     * PDOStatement::execute() is called.
     * -----------------------------------------------------------------------------------
     */
    foreach ($bindings as $key => $value) {
        $bound = $statement->bindValue(
                getInputParameterName($key)
                , $value
                , getInputParameterDataType($value)
        );

        if (!$bound) {
            throw new UnexpectedValueException('An input parameter can not be bound!');
        }
    }

    /*
     * Execute the prepared statement.
     * 
     * ------------------------------------------------------------------
     * PDOStatement::execute returns TRUE on success or FALSE on failure.
     * ------------------------------------------------------------------
     */
    $executed = $statement->execute();

    if (!$executed) {
        throw new UnexpectedValueException('The prepared statement could not be executed!');
    }

    /*
     * Fetch users list - array of objects.
     */
    $users = $statement->fetchAll(PDO::FETCH_OBJ);

    if ($users === FALSE) {
        throw new UnexpectedValueException('Fetching users list failed!');
    }

    /*
     * Close connection.
     */
    $connection = NULL;

    /*
     * Handle results.
     */
    if (empty($users)) {
        $response->getBody()->write(
            '{
                "error": {
                    "message":"Invalid"
                }
            }'
        );
    } else {
        $response->getBody()->write(json_encode($users));
    }
} catch (PDOException $exc) {
    echo $exc->getMessage();
    // $logger->log($exc);
    exit();
} catch (Exception $exc) {
    echo $exc->getMessage();
    // $logger->log($exc);
    exit();
}

/**
 * Get the name of an input parameter by its key in the bindings array.
 *  
 * @param int|string $key The key of the input parameter in the bindings array.
 * @return int|string The name of the input parameter.
 */
function getInputParameterName($key) {
    return is_int($key) ? ($key + 1) : (':' . ltrim($key, ':'));
}

/**
 * Get the PDO::PARAM_* constant, e.g the data type of an input parameter, by its value.
 *  
 * @param mixed $value Value of the input parameter.
 * @return int The PDO::PARAM_* constant.
 */
function getInputParameterDataType($value) {
    $dataType = PDO::PARAM_STR;
    if (is_int($value)) {
        $dataType = PDO::PARAM_INT;
    } elseif (is_bool($value)) {
        $dataType = PDO::PARAM_BOOL;
    }
    return $dataType;
}

编辑：

对于我的项目，我开发了一个DbAdapter类。方法名称是不言自明的。因此，每个网页内没有更多的spagetti代码：-)而是：

• sql语句，
• 绑定数组，
• 对数据库适配器中相应方法的调用，以及
• 从数据库断开连接线

您问题的解决方案如下所示：

<?php

//***********************************************************************************
// Put this in a php file (like db.php) to include whereever you need db data access.
//***********************************************************************************
//
// Db configs.
define('DB_HOST', '...');
define('DB_PORT', 3306);
define('DB_DBNAME', '...');
define('DB_CHARSET', 'utf8');
define('DB_USERNAME', '...');
define('DB_PASSWORD', '...');
define('DB_DRIVER_NAME', 'mysql');

// Create db adapter.
$dbAdapter = new DbAdapter(DB_HOST, DB_DBNAME, DB_USERNAME, DB_PASSWORD, DB_PORT, DB_CHARSET);

//***********************************************************************************

$userid1 = 1;

// Sql statement.
$sql = 'SELECT ... FROM ... WHERE subjects.userid = :userid1 GROUP BY ... ORDER BY ...';

// Input parameters.
$bindings = array(
    ':userid1' => $userid1,
);

// Fetch users.
$users = $dbAdapter->fetchAll($sql, $bindings);

// Disconnect from db.
$dbAdapter->disconnect();

/*
 * Handle results.
 */
if (empty($users)) {
    //...
} else {
    //...
}

调用的适配器方法是public：

• connect ：连接到数据库，例如创建PDO实例，例如创建db连接。
• 断开连接 ：与数据库断开连接。
• fetchAll ：一次获取更多记录。返回数组的数组。因此，每个元素都是对应于db记录的数组。
• fetchOne ：仅获取一条记录。
• fetchColumn ：获取列值。
• update ：执行UPDATE查询。返回受影响的行数。
• delete ：执行DELETE查询。返回受影响的行数。
• insert ：执行INSERT查询。返回最后的插入ID。
• getLastInsertId ：返回执行INSERT操作后的最后一个插入ID。

就这样 ：-）

<?php

/*
 * Database adapter.
 */

/**
 * Database adapter.
 */
class DbAdapter {

    /**
     * Connection configs.
     * 
     * @var array
     */
    private $connectionConfigs;

    /**
     * Database connection.
     * 
     * @var PDO
     */
    private $connection;

    /**
     * PDO statement.
     * 
     * @var PDOStatement
     */
    private $statement;

    /**
     * 
     * @param string $host [optional] Host.
     * @param string $dbname [optional] Database name.
     * @param string $username [optional] User name.
     * @param string $password [optional] Password.
     * @param string $port [optional] Port.
     * @param string $charset [optional] Character set.
     * @param string $driverName [optional] Driver name.
     * @param array $driverOptions [optional] Driver options.
     * @return string DSN string.
     */
    public function __construct($host = '', $dbname = ''
    , $username = '', $password = '', $port = 3306, $charset = 'utf8', $driverName = 'mysql'
    , $driverOptions = array(
        PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
        PDO::ATTR_EMULATE_PREPARES => FALSE,
        PDO::ATTR_PERSISTENT => TRUE,
    )) {
        $this->setConnectionConfigs(array(
            'host' => $host,
            'dbname' => $dbname,
            'username' => $username,
            'password' => $password,
            'port' => $port,
            'charset' => $charset,
            'driverName' => $driverName,
            'driverOptions' => $driverOptions,
        ));
    }

    /**
     * Connect to db, e.g. create a PDO instance.
     * 
     * @return $this
     * @throws PDOException
     */
    public function connect() {
        if (!isset($this->connection) || !$this->connection) {
            try {
                $this->connection = new PDO(
                        $this->createDsn(
                                $this->connectionConfigs['host']
                                , $this->connectionConfigs['dbname']
                                , $this->connectionConfigs['port']
                                , $this->connectionConfigs['charset']
                                , $this->connectionConfigs['driverName']
                        )
                        , $this->connectionConfigs['username']
                        , $this->connectionConfigs['password']
                        , $this->connectionConfigs['driverOptions']
                );
            } catch (PDOException $pdoException) {
                echo $pdoException->getMessage();
                exit();
            }
        }

        return $this;
    }

    /**
     * Disconnect from db.
     * 
     * @return $this
     */
    public function disconnect() {
        $this->connection = NULL;

        return $this;
    }

    /**
     * Create a DSN string.
     * 
     * @param string $host Host.
     * @param string $dbname Database name.
     * @param string $port Port.
     * @param string $charset Character set.
     * @param string $driverName Driver name.
     * @return string DSN string.
     */
    private function createDsn($host, $dbname, $port, $charset, $driverName) {
        switch ($driverName) {
            default: // mysql
                $dsn = sprintf('%s:host=%s;port=%s;dbname=%s;charset=%s'
                        , $driverName
                        , $host
                        , $port
                        , $dbname
                        , $charset
                );
                break;
        }

        return $dsn;
    }

    /**
     * Fetch data by executing a SELECT sql statement.
     * 
     * @param string $sql Sql statement.
     * @param array $bindings [optional] Input parameters.
     * @param integer $fetchMode [optional] Fetch mode for a PDO statement.
     *  Must be one of the PDO::FETCH_* constants.
     * @param mixed $fetchArgument [optional] Fetch argument for a PDO statement.
     * @param array $fetchConstructorArguments [optional] Constructor arguments for a PDO statement 
     *  when fetch mode is PDO::FETCH_CLASS.
     * @return array An array containing the rows in the result set, or FALSE on failure.
     * @throws UnexpectedValueException
     */
    public function fetchAll($sql, array $bindings = array(), $fetchMode = PDO::FETCH_ASSOC, $fetchArgument = NULL, array $fetchConstructorArguments = array()) {
        $this
                ->prepareStatement($sql)
                ->bindInputParameters($bindings)
                ->executePreparedStatement()
        ;

        try {
            if (isset($fetchArgument)) {
                $data = $this->getStatement()->fetchAll($fetchMode, $fetchArgument, $fetchConstructorArguments);
            } else {
                $data = $this->getStatement()->fetchAll($fetchMode);
            }

            if ($data === FALSE) {
                throw new UnexpectedValueException('Fetching data failed!');
            }

            return $data;
        } catch (Exception $exception) {
            echo $exception->getMessage();
            exit();
        }
    }

    /**
     * Fetch the next row from the result set by executing a SELECT sql statement.
     * The fetch mode property determines how PDO returns the row.
     * 
     * @param string $sql Sql statement.
     * @param array $bindings [optional] Input parameters.
     * @param integer $fetchMode [optional] Fetch mode for a PDO statement.
     *  Must be one of the PDO::FETCH_* constants.
     * @param integer $fetchCursorOrientation [optional] For a PDOStatement object representing 
     *  a scrollable cursor, this value determines which row will be returned to the caller.
     * @param integer $fetchCursorOffset [optional] The absolute number of the row in the result 
     *  set, or the row relative to the cursor position before PDOStatement::fetch() was called.
     * @return array An array containing the next row in the result set, or FALSE on failure.
     * @throws Exception
     */
    public function fetchOne($sql, array $bindings = array(), $fetchMode = PDO::FETCH_ASSOC, $fetchCursorOrientation = PDO::FETCH_ORI_NEXT, $fetchCursorOffset = 0) {
        $this
                ->prepareStatement($sql)
                ->bindInputParameters($bindings)
                ->executePreparedStatement()
        ;

        try {
            /*
             * =========================================================
             * NB:
             * =========================================================
             * PDOStatement::fetch returns FALSE not only on failure,
             * but ALSO when no record is found! This is a BUG. That's
             * why I made the try-catch block: maybe on failure will
             * throw an exception.
             * 
             * Instead, PDOStatement::fetchAll returns FALSE on failure,
             * but an empty array if no record is found. This is the
             * correct behaviour.
             * =========================================================
             */
            $data = $this->getStatement()->fetch($fetchMode, $fetchCursorOrientation, $fetchCursorOffset);

            return $data;
        } catch (Exception $exception) {
            echo $exception->getMessage();
            exit();
        }
    }

    /**
     * Returns a single column from the next row of a result set 
     * or FALSE if there are no more rows.
     * 
     * =================================================================
     * Note:
     * -----
     * PDOStatement::fetchColumn() should not be used to retrieve 
     * boolean columns, as it is impossible to distinguish a value 
     * of FALSE from there being no more rows to retrieve.
     * Use PDOStatement::fetch() instead.
     * 
     * Warning:
     * --------
     * There is no way to return another column from the same row if you
     * use PDOStatement::fetchColumn() to retrieve data.
     * =================================================================
     * 
     * @param string $sql Sql statement.
     * @param array $bindings [optional] Input parameters.
     * @param integer $columnNumber [optional] 0-indexed number of the 
     *  column you wish to retrieve from the row. If no value is supplied, 
     *  PDOStatement::fetchColumn() fetches the first column.
     * @return mixed A single column from the next row of a result set 
     *  or FALSE if there are no more rows.
     * @throws Exception
     */
    public function fetchColumn($sql, array $bindings = array(), $columnNumber = 0) {
        $this
                ->prepareStatement($sql)
                ->bindInputParameters($bindings)
                ->executePreparedStatement()
        ;

        try {
            return $this->getStatement()->fetchColumn($columnNumber);
        } catch (Exception $exception) {
            echo $exception->getMessage();
            exit();
        }
    }

    /**
     * Store data by executing an INSERT sql statement.
     * 
     * @param string $sql Sql statement.
     * @param array $bindings [optional] Input parameters.
     * @return int Last insert id.
     */
    public function insert($sql, array $bindings = array()) {
        $this
                ->prepareStatement($sql)
                ->bindInputParameters($bindings)
                ->executePreparedStatement()
        ;

        return $this->getLastInsertId();
    }

    /**
     * Update data by executing an UPDATE sql statement.
     * 
     * @param string $sql Sql statement.
     * @param array $bindings [optional] Input parameters.
     * @return int Number of affected rows.
     */
    public function update($sql, array $bindings = array()) {
        $this
                ->prepareStatement($sql)
                ->bindInputParameters($bindings)
                ->executePreparedStatement()
        ;

        return $this->getStatement()->rowCount();
    }

    /**
     * Delete data by executing a DELETE sql statement.
     * 
     * @param string $sql Sql statement.
     * @param array $bindings [optional] Input parameters.
     * @return int Number of affected rows.
     */
    public function delete($sql, array $bindings = array()) {
        $this
                ->prepareStatement($sql)
                ->bindInputParameters($bindings)
                ->executePreparedStatement()
        ;

        return $this->getStatement()->rowCount();
    }

    /**
     * Prepare and validate an sql statement.
     * 
     * ----------------------------------------------------
     * If the database server cannot successfully prepare 
     * the statement, PDO::prepare() returns FALSE or emits 
     * PDOException (depending on error handling settings).
     * ----------------------------------------------------
     * 
     * @param string $sql Sql statement.
     * @return $this
     * @throws PDOException
     * @throws UnexpectedValueException
     */
    private function prepareStatement($sql) {
        $this->connect();
        try {
            $statement = $this->getConnection()->prepare($sql);

            if (!$statement) {
                throw new UnexpectedValueException('The sql statement can not be prepared!');
            }

            $this->setStatement($statement);
        } catch (PDOException $pdoException) {
            echo $pdoException->getMessage();
            exit();
        } catch (Exception $exception) {
            echo $exception->getMessage();
            exit();
        }

        return $this;
    }

    /**
     * Bind the input parameters to a prepared PDO statement.
     * 
     * @param array $bindings Input parameters.
     * @return $this
     * @throws UnexpectedValueException
     */
    private function bindInputParameters($bindings) {
        foreach ($bindings as $key => $value) {
            try {
                $bound = $this->getStatement()->bindValue(
                        $this->getInputParameterName($key)
                        , $value
                        , $this->getInputParameterDataType($value)
                );

                if (!$bound) {
                    throw new UnexpectedValueException('A value can not be bound!');
                }
            } catch (Exception $exception) {
                echo $exception->getMessage();
                exit();
            }
        }

        return $this;
    }

    /**
     * Get the name of an input parameter by its key in the bindings array.
     *  
     * @param int|string $key The key of the input parameter in the bindings array.
     * @return int|string The name of the input parameter.
     */
    private function getInputParameterName($key) {
        return is_int($key) ? ($key + 1) : (':' . ltrim($key, ':'));
    }

    /**
     * Get the PDO::PARAM_* constant, e.g the data type of an input parameter, by its value.
     *  
     * @param mixed $value Value of the input parameter.
     * @return int The PDO::PARAM_* constant.
     */
    private function getInputParameterDataType($value) {
        $dataType = PDO::PARAM_STR;
        if (is_int($value)) {
            $dataType = PDO::PARAM_INT;
        } elseif (is_bool($value)) {
            $dataType = PDO::PARAM_BOOL;
        }
        return $dataType;
    }

    /**
     * Execute a prepared PDO statement.
     * 
     * @return $this
     * @throws UnexpectedValueException
     */
    private function executePreparedStatement() {
        try {
            if (!$this->getStatement()->execute()) {
                throw new UnexpectedValueException('The statement can not be executed!');
            }
        } catch (Exception $exception) {
            echo $exception->getMessage();
            exit();
        }

        return $this;
    }

    /**
     * Get the ID of the last inserted row or of the sequence value.
     * 
     * @param string $sequenceObjectName [optional] Name of the sequence object 
     *  from which the ID should be returned.
     * @return string The ID of the last row, or the last value retrieved from the specified 
     *  sequence object, or an error IM001 SQLSTATE If the PDO driver does not support this.
     * @throws PDOException
     */
    public function getLastInsertId($sequenceObjectName = NULL) {
        $this->connect();
        try {
            return $this->getConnection()->lastInsertId($sequenceObjectName);
        } catch (PDOException $pdoException) {
            echo $pdoException->getMessage();
            exit();
        }
    }

    /**
     * Get connection configs.
     * 
     * @return array
     */
    public function getConnectionConfigs() {
        return $this->connectionConfigs;
    }

    /**
     * Set connection configs.
     * 
     * @param array $connectionConfigs Connection configs.
     * @return $this
     */
    public function setConnectionConfigs($connectionConfigs) {
        $this->connectionConfigs = $connectionConfigs;
        return $this;
    }

    /**
     * Get database connection.
     * 
     * @return PDO Database connection.
     */
    public function getConnection() {
        return $this->connection;
    }

    /**
     * Set database connection.
     * 
     * @param PDO $connection Database connection.
     * @return $this
     */
    public function setConnection(PDO $connection) {
        $this->connection = $connection;
        return $this;
    }

    /**
     * Get PDO statement.
     * 
     * @return PDOStatement
     */
    public function getStatement() {
        return $this->statement;
    }

    /**
     * Set PDO statement.
     * 
     * @param PDOStatement $statement PDO statement.
     * @return $this
     */
    public function setStatement(PDOStatement $statement) {
        $this->statement = $statement;
        return $this;
    }

}