我怎么能在PHP中执行这个基本的表查询？

Suppose I have a table TABLE:

 NAME  ID   ...
 m    -1    ...
 f    -1    ...
 g    -1    ...
 b    -1    ...
 z    -1    ...

And I want to turn it into:

 NAME  ID   ...
 f     1    ...
 g     2    ...
 m     3    ...
 b    -1    ...
 z    -1    ...

You probably get the idea:

  1. select the first 3 rows from the original table (preserving order)
  2. order selected rows by the NAME column.
  3. update selected rows' IDs with their position in the new table (keeping the remaining unselected rows in their original positions).

So (m, f, g) got sorted to (f, g, m) and (b, z) remained (b, z).

Here's how I am trying to do it in PHP:

$count = 0;
$query = "UPDATE TABLE SET ID = $count:= $count + 1 ORDER by NAME DESC LIMIT 3";
mysqli_query($con, $query);

But I don't think I can just go ahead and increment a counter and store its value like that. Any advice?

You can try this :

$limit = 3;
for($count = 0 ; $count < $limit;$count++ ){
$query = "UPDATE TABLE SET ID = $count + 1 WHERE ID = '-1' ORDER by NAME DESC";
mysqli_query($con, $query);
}
$query = "UPDATE TABLE SET ID = '-1' WHERE ID > $limit ORDER by NAME DESC";
mysqli_query($con, $query);

In the above logic :

In the final loop, all the IDs are set to $limit However the update command outisde the loop will set back IDs to -1 again

First, you can quickly query for the first 3 rows in the table and get the name property only and assign the value in an array.

$sql = "select name from table order by name limit 3"
$query = $mysqli->query($sql);

Now let's construct a helper array:

while ($row = $mysqli->fetch_assoc()) {
    $a[] = $row['name'];
}

Now just structure the queries:

foreach($a as $id => $name) {
    $query = "update table set id={$id+1} where name='$name' limit 1";
     // execute the query
}

Note that I assume that the name is unique so I added the limit 1 directive to tell it stop looking for rows to update once it has found a row.

Also, don't forget that array keys are counting starting from 0, hence we are adding 1 to the $id in the loop.

There may be more elegant solutions but this one is rather easy to understand and use.

In MySQL:

SET @row_number = 0;

update TABLE d
join
    (
     select
          NAME,
          @row_number:=@row_number+1 as ID,
     from
          (select NAME from TABLE limit 3) t
     order by
          NAME asc
    ) s on s.NAME = d.NAME
set d.ID = s.ID;

SQLFiddle: http://sqlfiddle.com/#!9/dffecf/1

This assumes NAME is your unique key, otherwise likely best to replace with an Identity column in your table and use that for the update.

This approach may require some syntax changes depending on your DB engine. By doing this in SQL, we only make one pass at the DB. Not a huge deal to iterate in multiple passes with PHP if you're only updating three records, but if it was a 1000, etc.