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 MySqlGrammar
return $this->grammar->compileSelect($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/Grammar
public 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($table);
}
// Illuminate/Database/Grammar
public function wrapTable($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。
