从PHP中准备好的MySQL语句的结果计算唯一值

Question

I have an online calendar listing upcoming live music. I use a prepared statement to fetch the listings for the next eight days and display them in a table. What I need to do, before displaying the results, is count the number of unique dates ('Date') within the listings. For example, if only five days out of the next eight have events happening, I need to know that number is 5.

Using COUNT(DISTINCT) works for giving me that number, but then it only displays one row of results, so I need another solution

My code is this:

$mysqli = new mysqli("Login Stuff Here");
if ($mysqli->connect_errno){
  echo "Failed to connect to MySQL: (" . $mysqli->connect_errno . ") " . $mysqli->connect_error;
}

$start = strtotime('today midnight');
$stop = strtotime('+1 week');

$start = date('Y-m-d', $start);
$stop = date('Y-m-d', $stop);

$allDates = $mysqli->prepare("SELECT 
    ID,
    Host,
    Type,
    Bands,
    Date,
    Time,
    Price,
    Note,
    Zip,
    URL
    FROM things WHERE (Date >= ? AND Date <= ?) ORDER BY Date, Time, Host");
$allDates->bind_param("ss", $start, $stop);
$allDates->execute();
$allDates->bind_result($ID, $Host, $Type, $Bands, $Date, $Time, $Price, $Note, $Zip, $URL);

while($allDates->fetch()):
  // ECHO ALL THE INFO IN A NICE TABLE
  endwhile;

$allDates->close();

I need to count the unique values (and maybe even retrieve them) from the 'Date' column. Right now I have it working by doing a separate query, but I'm sure there's a better way.

EDIT: Ultimately, I wound up doing a separate query, which worked out well as I was able to use it for other things as well. I found that using GROUP BY always only returned just one result per date, so it didn't work for displaying the full listings. Maybe I was going at it wrong, but I wound up being good in another way. Thanks!

Answer 1

您缺少分组依据：

GROUP BY Date

Answer 2

To use any aggregate method like count, sum for specific group. You need to apply GROUP BY on specific column or list of columns.

Example :

 GROUP BY Date ORDER BY Date, Time, Host

Please note that, list of columns in SELECT MUST match the list of columns mentioned along with GROUP BY .

Also,

Date >= ? AND Date <= ?

can be replaced by

Date BETWEEN ? AND ?

Answer 3

I'd recommend grouping by date as you fetch the results from the query.

while($row = $allDates->fetch()) {
    $dates[$row['date'][] = $row;
}

That makes it easy to count the distinct dates

$count = count($dates);

And potentially simpler to format your output (headers/sections for each date, etc.)

foreach($dates as $date) {
    foreach($date as $event) {
        // ECHO ALL THE INFO IN A NICE TABLE
    }
}

It does require iterating the same data twice, but for a reasonable amount of data to display on a page, that shouldn't make much difference.