PHP处理多表查询时的SQL语句拆分与重新组装

在自己写框架时候会发现，多表查询组装SQL语句
<?php
$pre = "pre_";
$aid = "44";

$data = array("user.username,comment.uid,article.aid","user,comment,article","user.uid=comment.uid AND article.aid=comment.aid AND comment.aid=$aid");

//字段判断
if($data["0"] === "*"){
	$filed = "*";
}else{
	$arr = explode(",",$data["0"]);
	$length = count($arr);
	for($i=0; $i<$length; $i++){
		$filed .= $pre.$arr[$i].",";
	}
	$filed = substr($filed, 0,-1);
}

//表名判断
if(strpos($data["1"],",") > 1){
	$arr = explode(",",$data["1"]);
	$length = count($arr);
	for($i=0; $i<$length; $i++){
		$table .= $pre.$arr[$i].",";
	}
	$table = substr($table, 0,-1);
}else{
	$table = $pre.$data["1"];
}

//条件判断
$one = explode(" AND ", $data["2"]);
$len1 = count($one);
for($i=0; $i<$len1; $i++){
	$two[] = explode("=", $one[$i]);
}
$len2 = count($two);

for($i=0; $i<$len2; $i++){
	for($j=0; $j<$len2-1; $j++){
		if(strpos($two[$i][$j],".") > 1){
			$where .= $pre.$two[$i][$j]."=";
		}else{
			$where .= $two[$i][$j];
		}
	}
}
$where = explode("=",$where);

$len3 = count($where);

for($i=0; $i<$len3; $i++){
	if($i%2 == 0){
		$c .=$where[$i]."=";
	}else{
		$c .=$where[$i]." AND ";
	}
}
$where = substr($c, 0,-4);

echo $query = "SELECT $filed FROM $table WHERE $where";

?>