MySQLi multi_query（）好奇心

Question

Can someone explain me what does mysqli multi_query() is being used for? I don't understand wether a simple MySQLi query supports multimle statements or am I forced to use multi_query() in order to do a query like this:

   SET @row:=0;
   SELECT * FROM (
       select id,
       @row:=@row+1 AS `rank` FROM `users` WHERE `points` <= 63 ORDER BY `points` DESC
   ) AS number
   WHERE id = 1

I'm asking this becuase running $mysqli->query(ABOVE_STATEMENT) returns me a syntax error .. even though running the same statement in phpMyAdmin (set using mysqli extension) it succesfully returns the needed data.

Answer 1

You do not have to use multi_query to do that. The @row variable keeps its value for your whole session (ie until you disconnect from MySQL).

You may submit the SET statement and the SELECT statement as separate queries.

$mysqli->query("SET @row:=0");
$mysqli->query("SELECT * ... ");

There are no good reasons to use multi-query. There are good reasons not to use multi-query.

If someone tells you that it's more efficient to use multi-query because "one round trip is faster than two," tell them to show you a benchmark that proves it. Then tell them that you're not planning on running a thousand queries in one PHP request, because that's what it would take for the difference in performance to have any significant impact.

Answer 2

multi_query is if you are doing something such as:

select * from somewhere;
select id from somewhere;
select names from somewhere;

Check out http://www.w3schools.com/php/func_mysqli_multi_query.asp

Notice how you must store the first result, then call mysqli_next_result() to fetch the next results. It's throwing them all in 1 batch and returning them all back at you.

EDIT: Marc B is correct in the comment where you are technically using 2 statements by setting the row. In that case you are using multiple queries. I didn't see that at first.

Answer 3

mysqli_query indeed cannot execute your given queries.

If you want to use mysqli_multi_query , then you can do the following

$mysqli->multi_query(ABOVE_STATEMENT);
$mysqli->next_result(); # skip the first query result
$rs = $mysqli->store_result();
while ($row = $rs->fetch_row()) {
  ...
}