Group By and displaying entries under date

Question

Here's my data:

id date  
1 2009-01-01 10:15:23  
2 2009-01-01 13:21:29  
3 2009-01-02 01:03:13  
4 2009-01-03 12:20:19  
5 2009-01-03 13:01:06 

What I'd like to do is group by each date and then list the id numbers below each date. Should I be getting the distinct date values and then cycling through them to get the entries that fall on the date? is this efficient?

so my output would look like:

2009-01-01  
1  
2   
2009-01-02  
3  
2009-01-03  
4  
5  

Answer 1

$query = "SELECT DATE(date) as mydate, id FROM your_data_table ORDER BY mydate, id";
$result = $pdo->query($query);

$oldDate = false;
while ( list($date, $id) = $result->fetch(PDO::FETCH_NUM) ) {
    if ( $oldDate != $date ) {
        echo "$date\n$id\n";
        $oldDate = $date;
    } else {
        echo "$id\n";
    }
}

Instead of doing it in several queries, you just fetch all the data at once. If you only need some dates then you just put a WHERE clause before the ORDER BY.

Answer 2

Your stored dates have time information (the type of the column is DATETIME or TIMESTAMP), in order to do a group by, you need to extract only the date part your timespan, you can use the DATE() function.