PHP & Mysql，过滤掉 PHP-for 循环中的子查询到 MySQL 语句中

Question

我必须修改一个连接到极其混乱的数据库设计的 PHP 脚本。 解释这个问题的最好方法是发布代码，我将在其中描述通过代码块内的 cmets 发生的事情。加上一个简短的介绍：

短篇小说：
1.) 有一个“mainQuery”，在显示其数据的 for 循环内有 3 个嵌套子查询。
2.) 所有三个子查询都需要 main_table_ID 来计算每个 fetched_row 的出现次数。

我想要什么：
1.) 一个经过优化的 mainQuery，它单独计算 3 个 id 的出现次数。
2.) 因此，for 循环内根本没有子查询。

我通过优化 mainQuery 已经取得的成果：
1.) 我可以一次过滤一个子查询，请参阅语句：

SELECT
       main_table.id_col,
       count(table_1.a_col) AS count_1,
       main_table.a_col,
       main_table.b_col,
       main_table.c_col,
       main_table.d_col,
       main_table.e_col

FROM   main_table
LEFT JOIN
       table_1
ON
       table_1.a_col = main_table.id_col
GROUP BY
       main_table.id_col

但是当我在 for 循环中为其他嵌套语句之一添加另一个 LEFT JOIN 时，该语句将所有内容相加，并且 count_1 到 count_3 看起来相同。

现在是包含子查询和 for 循环的代码块：

<?php
$mainQuery = "SELECT 
                          main_table.id_col, 
                          main_table.a_Col, 
                          main_table.b_Col, 
                          main_table.c_col, 
                          main_table.d_col, 
                          main_table.e_col 
                   FROM 
                          main_table";

$mainQueryResult = mysql_query($mainQuery);
$mainQueryRows = mysql_num_rows($mainQueryResult);

for($j = 0 ; $j < $mainQueryRows ; ++$j){
    //mainQuery data
    $mainQueryRow = mysql_fetch_row($mainQueryResult);
    $main_table_ID = $mainQueryRow[0];

    //first subquery, can be filtered out one at a time 
    //using introductory-statement
    $count_1_subQuery = "SELECT * FROM table_1 
                         WHERE table_1.id_col = '$main_table_ID'";
    //first subquery-count of rows with matching id
    $count_1 = mysql_num_rows(mysql_query($count_1_SubQuery, $connection));

    //second subquery
    $count_2_subquery = "SELECT * FROM table_2 
             WHERE table_2.id_col = '$main_table_ID'";
    //second subquery-count of rows with matching id
    $count_2 = mysql_num_rows(mysql_query($count_2_subQuery, $connection));

    //third subquery
    $count_3_subquery = "SELECT * FROM table_3 
             WHERE table_3.id_col = '$main_table_ID'";
    //third subquery-count of rows with matching id
    $count_3 = mysql_num_rows(mysql_query($count_3_subQuery, $connection));

    //Fill Table with count_1 to count_3 
        //and main_table.a_col to main_table.e_col
    //no problems here
}
?>

我希望我能足够清楚地解释这个问题，我也希望你们中的一个可以帮助我，如果他/她可以分配时间来解决这样的问题。

Answer 1

首先GROUP BY 在子查询中，然后JOIN 到主表：

SELECT  
      m  . id_col
    , g1 . count_1  
    , g2 . count_2  
    , g3 . count_3  
    , m  . a_col  
    , m  . b_col  
    , m  . c_col  
    , m  . d_col  
    , m  . e_col      
FROM 
        main_table AS m
    LEFT JOIN 
        ( SELECT id_col
              , COUNT(*) AS count_1  
          FROM table_1  
          GROUP BY id_col 
        ) AS g1
      ON g1.id_col = m.id_col
    LEFT JOIN 
        ( SELECT id_col
               , COUNT(*) AS count_2  
          FROM table_2 
          GROUP BY id_col 
        ) AS g2
      ON g2.id_col = m.id_col
    LEFT JOIN 
        ( SELECT id_col
               , COUNT(*) AS count_3  
          FROM table_3  
          GROUP BY id_col 
        ) AS g3
      ON g3.id_col = m.id_col

Answer 2

SELECT  
  main_table.id_col,  
  count(DISTINCT t1.id_col) AS count_1,  
  count(DISTINCT t2.id_col) AS count_2,  
  count(DISTINCT t3.id_col) AS count_3,  
  main_table.a_col,  
  main_table.b_col,  
  main_table.c_col,  
  main_table.d_col,  
  main_table.e_col      
FROM  main_table m
LEFT JOIN table_1 t1 ON (t1.id_col = m.id_col)
LEFT JOIN table_2 t2 ON (t2.id_col = m.id_col)
LEFT JOIN table_3 t3 ON (t3.id_col = m.id_col)
GROUP BY m.id_col