Question

I need to pull several rows from a table and process them in two ways:

• aggregated on a key
• row-by-row, sorted by the same key

The table looks roughly like this:

table (
   key,
   string_data,
   numeric_data
)

So I’m looking at two approaches to the function I’m writing.

The first would pull the aggregate data with one query, and then query again inside a loop for each set of row-by-row data (the following is PHP-like pseudocode):

$rows = query(
        "SELECT key,SUM(numeric_data)
         FROM table
         GROUP BY key"
    );

foreach ($rows as $row) {
    <process aggregate data in $row>

    $key = $row['key'];
    $row_by_row_data = handle_individual_rows($key);
}

function handle_individual_rows($key)
{
    $rows = query(
            "SELECT string_data
             FROM table WHERE key=?",
            $key
        );

    <process $rows one row at a time>

    return $processed_data;
}

Or, I could do one big query and let the code do all the work:

$rows = query(
    "SELECT key, string_data, numeric_data
     FROM table"
);

foreach ($rows as $row) {
    <process rows individually and calculate aggregates as I go>
}

Performance is not a practical concern in this application; I’m just looking to write sensible and maintainable code.

I like the first option because it’s more modular — and I like the second option because it seems structurally simple. Is one option better than the other or is it really just a matter of style?

Answer 1

One SQL query, for sure.

This will

• Save you lots of roundtrips to database
• Allow to use more efficient GROUP BY methods

Since your aggregates may be performed equally well by the database, it will also be better for mainainability: you have all your resultset logic in one place.

Here is an example of a query that returns every row and calculates a SUM:

SELECT  string_data, numeric_data, SUM(numeric_data) OVER (PARTITION BY key)
FROM    table

Note that this will most probably use parallel access to calculate SUM‘s for different key‘s, which is hardly implementable in PHP.

Same query in MySQL:

SELECT  key, string_data, numeric_data,
        (
        SELECT  SUM(numeric_data)
        FROM    table ti
        WHERE   ti.key = to.key
        ) AS key_sum
FROM    table to