php - SQL SELECT 和 PHP 检索使用单表 VS 连接两个表的速度？

Question

我正在创建一个表然后显示记录。我想以“inch”格式显示用户高度，但它将以“cm”格式存储在用户表中。 现在我想知道哪种方式会给我更快的结果？

使用连接

// to select records  
 SELECT *.u FROM, h.height_inch user as u join height as h on u.height_cm =h.height_cm 
 WHERE u.user_id = 2 AND ...;
//for display in php  
 foreach($all_records AS $key => $val) {
  echo $val['height_inch'];
  }

OR 单用户表(快于/慢于):

 // to select records  
 SELECT * user WHERE u.user_id = 2 AND ...;
//for display in php  
 foreach($all_records AS $key => $val) {
   $height_in_cm = $val['height_inch']
   $height_in_inch= convert_to_inch($height_in_cm); //function for  convert height cm to inch..
    echo $height_in_inch;
  }

Answer 1

一种更快的替代方法是在 SQL 中简单地执行算术运算，这样您就不必存储固定的转换数据或迭代结果。

SELECT height_cm, height_inches as (height_cm / 2.54) FROM table;