Question

I have to adapt a PHP-script that is connected to a extremely messy database-design.
The best way to explain the problem is to post the code, within which im going to describe whats happening through comments inside the code-block. Plus a short introduction:

Short Story:
1.) There is a “mainQuery” and inside the for-loop that displays its data there are 3 nested subqueries.
2.) All three subqueries need the main_table_ID in order to count the occurences of each fetched_row.

What i want:
1.) An optimzied mainQuery that counts 3 occurences of an id separately on its own.
2.) Thus no subqueries at all inside the for loop.

What i have already achieved via optimizing the mainQuery:
1.) I can filter a subquery one at a time by, see statement:

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

But when i add another LEFT JOIN for the one of other nested statements inside the for-loop, the statement sums everything up and count_1 to count_3 look the same.

And now the code-block containing the subqueries and the for-loop:

<?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
}
?>

I hope i could explain the problem clearly enough and i also hope one of you guys can help me, if he/she can allot time for such a question.

Answer 1

First GROUP BY in subqueries, then JOIN to the main table:

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