说明：本文主要学习下Query Builder编译Fluent Api为SQL的细节和执行SQL的过程。实际上，上一篇聊到了\Illuminate\Database\Query\Builder这个非常重要的类，这个类含有三个主要的武器：MySqlConnection, MySqlGrammar, MySqlProcessor。MySqlConnection主要就是在执行SQL时做连接MySql数据库操作，MySqlProcessor主要就是用来对执行SQL后的数据集做后置处理操作，这两点已经在之前上篇聊过，那MySqlGrammar就是SQL语法编译器，用来编译Fluent Api为SQL。最后使用MySqlConnection::select($sql, $bindings)执行SQL。

开发环境：Laravel5.3 + PHP7

Builder::toSql()

看下toSql()的源码：

    public function toSql(){// $this->grammar = new MySqlGrammarreturn $this->grammar->compileSelect($this);}public function compileSelect(Builder $query){$sql = parent::compileSelect($query);// 从上一篇文章知道，$unions属性没有存储值，$wheres属性是有值的if ($query->unions) {$sql = '('.$sql.') '.$this->compileUnions($query);}return $sql;}

这里首先会调用Illuminate\Database\Query\GrammarsGrammar::compileSelect(Builder $query)，看下compileSelect(Builder $query)的源码：

    public function compileSelect(Builder $query){// $original = ['*']$original = $query->columns;if (is_null($query->columns)) {$query->columns = ['*'];}$sql = trim($this->concatenate($this->compileComponents($query)));$query->columns = $original;// $sql = 'select * from users where id = ?'return $sql;}protected $selectComponents = ['aggregate','columns','from','joins','wheres','groups','havings','orders','limit','offset','lock',];protected function compileComponents(Builder $query){$sql = [];foreach ($this->selectComponents as $component) {// if (! is_null($query->$component)) {$method = 'compile'.ucfirst($component);// 1. compileColumns($builder, ['*']) -> 'select ' . $this->columnize(['*'])// 2. compileFrom($builder, 'users'); -> 'from '.$this->wrapTable('users')// 3. compileWheres($builder, [ 0 => ['type' => 'basic', 'column' => 'id', 'operator' => '=', 'value' => 1, 'boolean' => 'and'], ])// $sql = ['columns' => 'select *', 'from' => 'from users', 'wheres' => 'where id = ?']$sql[$component] = $this->$method($query, $query->$component);}}return $sql;}

从上文源码中可知道，首先依次遍历片段集合:aggregate,columns,from,joins,wheres,groups,havings,orders,limit,offset,lock，查看属性有无存储值。在上文中知道，在片段$columns,from,wheres存有值为['*'], 'users', [['type' => 'basic', 'column' => 'id', 'operator' => '=', 'value' => 1, 'boolean' => 'and']]，然后通过拼接字符串调用方法compileColumns($builder, ['*']), compileFrom($builder, 'users'), compileWheres($builder, array)，依次看下这些方法的源码：

    protected function compileColumns(Builder $query, $columns){   if (! is_null($query->aggregate)) {return;}// $select = 'select '$select = $query->distinct ? 'select distinct ' : 'select ';return $select.$this->columnize($columns);}// Illuminate/Database/Grammarpublic function columnize(array $columns){// 依次经过wrap()函数封装下return implode(', ', array_map([$this, 'wrap'], $columns));}public function wrap($value, $prefixAlias = false){if ($this->isExpression($value)) {return $this->getValue($value);}if (strpos(strtolower($value), ' as ') !== false) {$segments = explode(' ', $value);if ($prefixAlias) {$segments[2] = $this->tablePrefix.$segments[2];}return $this->wrap($segments[0]).' as '.$this->wrapValue($segments[2]);}$wrapped = [];$segments = explode('.', $value);// $segments = ['*']foreach ($segments as $key => $segment) {if ($key == 0 && count($segments) > 1) {$wrapped[] = $this->wrapTable($segment);} else {// $wrapped = ['*']$wrapped[] = $this->wrapValue($segment);}}return implode('.', $wrapped);}protected function wrapValue($value){if ($value === '*') {return $value;}return '"'.str_replace('"', '""', $value).'"';}

通过源码很容易知道compileColumns($builder, ['*'])返回值select "*"，然后将该值以key-value形式存储在$sql变量中，这时$sql = ['columns' => 'select "*"']。
OK，看下compileFrom($builder,'users')源码：

    protected function compileFrom(Builder $query, $table){return 'from '.$this->wrapTable($table);}// Illuminate/Database/Grammarpublic function wrapTable($table){if ($this->isExpression($table)) {return $this->getValue($table);}// 返回"users"return $this->wrap($this->tablePrefix.$table, true);}

很容易知道返回值是from "users"，然后将该值存储在$sql变量中，这时$sql = ['columns' => 'select "*"', 'from' => 'from "users"']。OK,看下compileWheres($builder, array)的源码：

    protected function compileWheres(Builder $query){$sql = [];if (is_null($query->wheres)) {return '';}foreach ($query->wheres as $where) {$method = "where{$where['type']}"; // 'whereBasic'// 'and ' . $this->whereBasic($builder, ['type' => 'basic', 'column' => 'id', 'operator' => '=', 'value' => 1, 'boolean' => 'and']// -> $sql = ['and id = ?', ];$sql[] = $where['boolean'].' '.$this->$method($query, $where);}if (count($sql) > 0) {$sql = implode(' ', $sql);// $conjunction = 'where'$conjunction = $query instanceof JoinClause ? 'on' : 'where';// 去除掉'and'字符后为'where id = ?'return $conjunction.' '.$this->removeLeadingBoolean($sql);}return '';}protected function whereBasic(Builder $query, $where){// $value = '?'$value = $this->parameter($where['value']);// 返回'id = ?'return $this->wrap($where['column']).' '.$where['operator'].' '.$value;}

从源码中可知道返回值为where id = ?，这时$sql = ['columns' => 'select "*"', 'from' => 'from "users"', 'wheres' => 'where id = ?']。

OK, 最后通过concatenate()函数把$sql值拼接成字符串select "*" from "users" where id = ?：

    protected function concatenate($segments){return implode(' ', array_filter($segments, function ($value) {return (string) $value !== '';}));}

也就是说，通过SQL语法编译器MySqlGrammar把table('users')->where('id', '=', 1)编译成了SQL语句select * from users where id = ?。

MySqlConnection::select()

上文聊到Builder::runSelect()调用了三个方法：MySqlConnection::select(), Builder::toSql(), Builder::getBindings()，其中Builder::toSql()通过SQL语法编译器已经编译得到了SQL语句，Builder::getBindings()获取存储在$bindings[ ]的值。最后看下MySqlConnection::select()是如何执行SQL语句的：

    public function select($query, $bindings = [], $useReadPdo = true){// Closure就是用来执行SQL,并把$query = 'select * from users where id =?', $bindings = 1作为参数传递进去return $this->run($query, $bindings, function (Connection $me, $query, $bindings) use ($useReadPdo) {if ($me->pretending()) {return [];}// $statement = PDO::prepare('select * from users where id =?')/** @var \PDOStatement $statement */$statement = $this->getPdoForSelect($useReadPdo)->prepare($query);$me->bindValues($statement, $me->prepareBindings($bindings));//PDO三步走: SQL编译prepare() => 值绑定bindValue() => SQL执行execute()// PDO通过这种方式防止SQL注入$statement->execute();$fetchMode = $me->getFetchMode();$fetchArgument = $me->getFetchArgument();$fetchConstructorArgument = $me->getFetchConstructorArgument();if ($fetchMode === PDO::FETCH_CLASS && ! isset($fetchArgument)) {$fetchArgument = 'StdClass';$fetchConstructorArgument = null;}// PDOStatement::fetchAll(PDO::FETCH_OBJ);return isset($fetchArgument)? $statement->fetchAll($fetchMode, $fetchArgument, $fetchConstructorArgument): $statement->fetchAll($fetchMode);});}protected function run($query, $bindings, Closure $callback){$this->reconnectIfMissingConnection();$start = microtime(true);try {// 执行闭包函数$result = $this->runQueryCallback($query, $bindings, $callback);} catch (QueryException $e) {if ($this->transactions >= 1) {throw $e;}$result = $this->tryAgainIfCausedByLostConnection($e, $query, $bindings, $callback);}$time = $this->getElapsedTime($start);$this->logQuery($query, $bindings, $time);return $result;}protected function runQueryCallback($query, $bindings, Closure $callback){try {// 执行闭包函数$result = $callback($this, $query, $bindings);}catch (Exception $e) {throw new QueryException($query, $this->prepareBindings($bindings), $e);}return $result;}

通过源码知道主要是执行闭包来实现连接数据库和执行SQL操作，其中$statement = $this->getPdoForSelect($useReadPdo)->prepare($query)这句代码实现了数据库的连接操作和SQL语句送入MySQL服务器进行语句编译。上文中提前聊了通过数据库连接器MySqlConnector::connect()连接数据库，这里知道实际上连接数据库是在这个时刻才触发的，Laravel5.0版本好像还没有这么写：

    protected function getPdoForSelect($useReadPdo = true){return $useReadPdo ? $this->getReadPdo() : $this->getPdo();}public function getPdo(){if ($this->pdo instanceof Closure) {// 连接数据库，获得PDO实例return $this->pdo = call_user_func($this->pdo);}return $this->pdo;}

通过源码知道执行SQL操作很简单，就是常见的PDO操作：PDO三步走: SQL编译PDO::prepare() => 值绑定PDOStatement::bindValue() => SQL执行PDOStatement::execute()。所以这里可看出Query Builder是在PHP PDO的基础上实现的一层封装，使得用更加面向对象的Fluent API来操作数据库，而不需要写一行SQL语句。

OK, 总的来说，通过了解Query Builder的实现原理后，知道其并不复杂或神秘，只是一个对PDO更友好封装的包裹，Query Builder有几个重要的类或概念：连接类MySqlConnection及其为其服务的连接器MySqlConnector;Builder 类;SQL语法解析器MySqlGrammar;后置处理器MySqlProcessor。

OK, illuminate/database package不仅提供了Query Builder，还提供了Eloquent ORM。那Eloquent ORM又是什么，与Query Builder是什么关系呢？既然有了Query Builder，为何还提供了Eloquent ORM呢？
实际上，Eloquent ORM又是对Query Builder的封装，这样可以实现更多好用且Query Builder所没有的功能，如Model Relationships;Accessor/Mutator;Scopes等等。以后再聊Eloquent ORM的实现原理吧。

总结：本文主要学习了Query Builder编译SQL细节和执行SQL逻辑。后续在分享下Eloquent ORM的实现原理，到时见。