說明：本文主要學(xué)習(xí)下Query Builder編譯Fluent Api為SQL的細節(jié)和執(zhí)行SQL的過程。實際上，上一篇聊到了\Illuminate\Database\Query\Builder這個非常重要的類，這個類含有三個主要的武器：MySqlConnection, MySqlGrammar, MySqlProcessor。MySqlConnection主要就是在執(zhí)行SQL時做連接MySql數(shù)據(jù)庫操作，MySqlProcessor主要就是用來對執(zhí)行SQL后的數(shù)據(jù)集做后置處理操作，這兩點已經(jīng)在之前上篇聊過，那MySqlGrammar就是SQL語法編譯器，用來編譯Fluent Api為SQL。最后使用MySqlConnection::select($sql, $bindings)執(zhí)行SQL。

開發(fā)環(huán)境：Laravel5.3 + PHP7

Builder::toSql()

看下toSql()的源碼：

public function toSql(){// $this->grammar = new MySqlGrammarreturn $this->grammar->compileSelect($this);}public function compileSelect(Builder $query){$sql = parent::compileSelect($query);// 從上一篇文章知道，$unions屬性沒有存儲值，$wheres屬性是有值的if ($query->unions) {$sql = '('.$sql.') '.$this->compileUnions($query);}return $sql;}

這里首先會調(diào)用Illuminate\Database\Query\GrammarsGrammar::compileSelect(Builder $query)，看下compileSelect(Builder $query)的源碼：

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;}

從上文源碼中可知道，首先依次遍歷片段集合:aggregate,columns,from,joins,wheres,groups,havings,orders,limit,offset,lock，查看屬性有無存儲值。在上文中知道，在片段$columns,from,wheres存有值為['*'], 'users', [['type' => 'basic', 'column' => 'id', 'operator' => '=', 'value' => 1, 'boolean' => 'and']]，然后通過拼接字符串調(diào)用方法compileColumns($builder, ['*']), compileFrom($builder, 'users'), compileWheres($builder, array)，依次看下這些方法的源碼：

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 $columns){// 依次經(jīng)過wrap()函數(shù)封裝下return implode(', ', array_map([$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).'"';}

通過源碼很容易知道compileColumns($builder, ['*'])返回值select "*"，然后將該值以key-value形式存儲在$sql變量中，這時$sql = ['columns' => 'select "*"']。
OK，看下compileFrom($builder,'users')源碼：

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"，然后將該值存儲在$sql變量中，這時$sql = ['columns' => 'select "*"', 'from' => 'from "users"']。OK,看下compileWheres($builder, array)的源碼：

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'字符后為'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;}

從源碼中可知道返回值為where id = ?，這時$sql = ['columns' => 'select "*"', 'from' => 'from "users"', 'wheres' => 'where id = ?']。

OK, 最后通過concatenate()函數(shù)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 !== '';}));}

也就是說，通過SQL語法編譯器MySqlGrammar把table('users')->where('id', '=', 1)編譯成了SQL語句select * from users where id = ?。

MySqlConnection::select()

上文聊到Builder::runSelect()調(diào)用了三個方法：MySqlConnection::select(), Builder::toSql(), Builder::getBindings()，其中Builder::toSql()通過SQL語法編譯器已經(jīng)編譯得到了SQL語句，Builder::getBindings()獲取存儲在$bindings[ ]的值。最后看下MySqlConnection::select()是如何執(zhí)行SQL語句的：

public function select($query, $bindings = [], $useReadPdo = true){// Closure就是用來執(zhí)行SQL,并把$query = 'select * from users where id =?', $bindings = 1作為參數(shù)傳遞進去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編譯prepare() => 值綁定bindValue() => SQL執(zhí)行execute()// PDO通過這種方式防止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();$start = microtime(true);try {// 執(zhí)行閉包函數(shù)$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 $callback){try {// 執(zhí)行閉包函數(shù)$result = $callback($this, $query, $bindings);}catch (Exception $e) {throw new QueryException($query, $this->prepareBindings($bindings), $e);}return $result;}

通過源碼知道主要是執(zhí)行閉包來實現(xiàn)連接數(shù)據(jù)庫和執(zhí)行SQL操作，其中$statement = $this->getPdoForSelect($useReadPdo)->prepare($query)這句代碼實現(xiàn)了數(shù)據(jù)庫的連接操作和SQL語句送入MySQL服務(wù)器進行語句編譯。上文中提前聊了通過數(shù)據(jù)庫連接器MySqlConnector::connect()連接數(shù)據(jù)庫，這里知道實際上連接數(shù)據(jù)庫是在這個時刻才觸發(fā)的，Laravel5.0版本好像還沒有這么寫：

protected function getPdoForSelect($useReadPdo = true){return $useReadPdo ? $this->getReadPdo() : $this->getPdo();}public function getPdo(){if ($this->pdo instanceof Closure) {// 連接數(shù)據(jù)庫，獲得PDO實例return $this->pdo = call_user_func($this->pdo);}return $this->pdo;}

通過源碼知道執(zhí)行SQL操作很簡單，就是常見的PDO操作：PDO三步走: SQL編譯PDO::prepare() => 值綁定PDOStatement::bindValue() => SQL執(zhí)行PDOStatement::execute()。所以這里可看出Query Builder是在PHP PDO的基礎(chǔ)上實現(xiàn)的一層封裝，使得用更加面向?qū)ο蟮腇luent API來操作數(shù)據(jù)庫，而不需要寫一行SQL語句。

OK, 總的來說，通過了解Query Builder的實現(xiàn)原理后，知道其并不復(fù)雜或神秘，只是一個對PDO更友好封裝的包裹，Query Builder有幾個重要的類或概念：連接類MySqlConnection及其為其服務(wù)的連接器MySqlConnector;Builder 類;SQL語法解析器MySqlGrammar;后置處理器MySqlProcessor。

OK, illuminate/database package不僅提供了Query Builder，還提供了Eloquent ORM。那Eloquent ORM又是什么，與Query Builder是什么關(guān)系呢？既然有了Query Builder，為何還提供了Eloquent ORM呢？
實際上，Eloquent ORM又是對Query Builder的封裝，這樣可以實現(xiàn)更多好用且Query Builder所沒有的功能，如Model Relationships;Accessor/Mutator;Scopes等等。以后再聊Eloquent ORM的實現(xiàn)原理吧。

總結(jié)：本文主要學(xué)習(xí)了Query Builder編譯SQL細節(jié)和執(zhí)行SQL邏輯。后續(xù)在分享下Eloquent ORM的實現(xiàn)原理，到時見。

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