Suppose i have 1kk records in my database.
Now i need to select some data, and also i need to know how many fields did i select, so my question is:
Is it better to run one query to count data like this:
SELECT COUNT("id") from table where something = 'something'
And after that run one more querio for selection like this:
SELECT 'some_field' from table where something = 'something';
Or Maybe it's better to just select data and then just count it with php like:
count($rows);
Or maybe there is even better ways to do it, for example do it all in one query?
Reading between the lines, I think what your are probably after is SQL_CALC_FOUND_ROWS. This allows you to select part of a result set (using a LIMIT clause), and still calculate the total number of matching rows in a single operation. You still use two queries, but the actual search operation in the data only happens once:
// First get the results you want...
$result = mysql_query("
SELECT SQL_CALC_FOUND_ROWS
FROM `table`
WHERE `something` = 'something'
LIMIT 0, 10
");
// ...now get the total number of results
$numRows = mysql_query("
SELECT FOUND_ROWS()
");
$numRows = mysql_fetch_row($numRows);
$numRows = $numRows[0];
If you fetch all that 1000 records then you can count while you are fetching:
$res=mysql_query("SELECT 'some_field' from table where something = 'something'");
while($r = mysql_fetch_*($res)) {
$count++;
//> Do stuff
}
This way you make only one query and you don't use mysql_num_rows();
One query would be:
SELECT Count(*) AS NumRows, some_field from table
GROUP BY some_field where something = 'something';
Related
So I had a pretty hard time describing this problem in the title, but basically I need to be able to get the number of rows my SQL query returns from the WHERE clause but is not limited by the LIMIT clause.
Example:
I have a table consisting of 10 posts. They all have column "show" = "true".
I then write my query like this:
$result = "SELECT * FROM table WHERE show = 'true' LIMIT 5";
I now need to get a variable on how many posts would be returned, had the LIMIT not been there.
$count = mysqli_num_rows($result); does not work as it'll only tell me 5 because of the LIMIT. I somehow need to know that there's 10 posts with column "show" = "true", even though my query only returns 5 posts because of the LIMIT.
Help would be very much appreciated!
Use SQL_CALC_FOUND_ROWS and FOUND_ROWS():
select SQL_CALC_FOUND_ROWS t.*
from table t.*
where show = 'true'
limit 5;
Then:
select found_rows();
The documentation is here.
You can use SQL_CALC_FOUND_ROWS and FOUND_ROWS():
SELECT SQL_CALC_FOUND_ROWS * FROM table WHERE show = 'true' LIMIT 5;
SELECT FOUND_ROWS();
http://dev.mysql.com/doc/refman/5.1/en/information-functions.html#function_found-rows
Another option is to issue two distinct queries, one to get the first five (as you have already), and another to get the total count:
SELECT * FROM table WHERE show = 'true' LIMIT 5;
SELECT COUNT(*) FROM table WHERE show = 'true';
I am using a script that has a different way of doing a mySQL query to what I am used to. It starts with:
$query = $db->query("SELECT * etc ..... ");
then
while ($result = $db->fetchArray($query)) {
with variables shown as $result['a'], $result['b']. etc.
All I want to do is count the rows that are selected by the query, but mysql_num_rows doesn't work on $result.
What can I use instead?
You can use the count function to count the rows
$query = $db->query("SELECT count(*) as count from (SELECT * etc ..... ) as sq ");
$result = $db->fetchArray($query);
echo $result['count'];
You can change the query to:
SELECT count(*) as cnt etc .....
Then read the results back from the query.
To get the total number of records, I usually use this query:
$total= mysql_num_rows(mysql_query("SELECT id FROM t_statistic WHERE pageid = $pid"));
but I got one the other query like below:
$data = mysql_fetch_object(mysql_query("SELECT COUNT(id) AS num_rows FROM t_statistic WHERE pageid = $pid"));
$total = $data->num_rows;
Between the two queries above. Which is more quickly and effectively (when the total number of records in the millions)?
I prefer the second query. It gives you already the record count, while the first query gives you the list of IDs (not the count), although it has been filtered but there are some cases when ID exist more than once in the table.
The Second query is quick and efficient:
SELECT COUNT(id) AS num_rows FROM t_statistic WHERE pageid = $pid
If you know about query optimisation. The query will only keeps only count in memory while calculating the answer. And directly gives number of rows.
Where as first query:
SELECT id FROM t_statistic WHERE pageid = $pid
Keeps all the selected rows in memory. then number of rows are calculated in further operation.
So second query is best in both ways.
Definitely the second one.
Some engines, like MySQL can do a count just by looking at an index rather than the table's data.
I've used something like the following on databases with millions of records.
SELECT count(*) as `number` FROM `table1`;
Way faster than: mysql_num_rows($res);
BTW: The * in Count(*) basically means it won't look at the data, it will just count the records, as opposed to Count(colname).
1) SELECT COUNT(*) FROM t_statistic WHERE pageid = $pid" --> count(*) counts all rows
2)SELECT COUNT(id) FROM t_statistic WHERE pageid = $pid" --> COUNT(column) counts non-NULLs only
3) SELECT COUNT(1) FROM t_statistic WHERE pageid = $pid" -->COUNT(1) is the same as COUNT(*) because 1 is a non-null expressions
Your use of COUNT(*) or COUNT(column) should be based on the desired output only.
So. Finally we have result is count(column) is more faster compare to count(*) .
I have a table as below,
ID Name Age
----------------------
100 A 10
203 B 20
Now how do i select only row1 using MySQL SELECT command and then I've to increase +1 to it to select row2. In short I'll be using for loop to do certain operations.
Thanks.
Sounds like you've got a mix up. You want to select all the rows you want to iterate through in your for loop with your query, and then iterate through them one by one using php's mysql functions like mysql_fetch_row
You should not try to use tables in a linear fashion like this. Set your criteria, sorting as appropriate, and then to select the next row use your existing criteria and limit it to one row.
SELECT * FROM `table` ORDER BY `ID` LIMIT 1
SELECT * FROM `table` ORDER BY `ID` WHERE ID > 100 LIMIT 1
You'd probably be better off retrieving all rows that you need, then using this. Note the LIMIT is entirely optional.
$query = mysql_query(' SELECT ID, Name, Age FROM table_name WHERE condition LIMIT max_number_you_want '))
while ($row = mysql_fetch_assoc($query)
{
// Do stuff
// $row['ID'], $row['Name'], $row['Age']
}
Lots of small queries to the database will execute much slower than one decent-sized one.
You should get the result into an array (php.net : mysql_fetch_*).
And after you'll can loop on the array "to do certain operations"
Yep, this is a pretty common thing to do in PHP. Like the others who have posted, here is my version (using objects instead of arrays):
$result = mysql_query("SELECT * FROM table_name");
while ($row = mysql_fetch_object($result)) {
// Results are now in the $row variable.
// ex: $row->ID, $row->Name, $row->Age
}
If I need to know the total number of rows in a table of database I do something like this:
$query = "SELECT * FROM tablename WHERE link='1';";
$result = mysql_query($query);
$count = mysql_num_rows($result);
Updated: I made a mistake, above is my actual way. I apologize to all
So you see the total number of data is recovered scanning through the entire database.
Is there a better way?
$query = "SELECT COUNT(*) FROM tablename WHERE link = '1'";
$result = mysql_query($query);
$count = mysql_result($result, 0);
This means you aren't transferring all your data between the database and PHP, which is obviously a huge waste of time and resources.
For what it's worth, your code wouldn't actually count the number of rows - it'd give you 2x the number of columns, as you're counting the number of items in an array representing a single row (and mysql_fetch_array gives you two entries in the array per column - one numerical and one for the column name)
SELECT COUNT(*) FROM tablename WHERE link='1';
You could just do :
SELECT count(*) FROM tablename;
for your query. The result will be a single column containing the number of rows.
If I need to know the total number of rows in a table of database
Maybe I'm missing something here but if you just want to get the total number of rows in a table you don't need a WHERE condition. Just do this:
SELECT COUNT(*) FROM tablename
With the WHERE condition you will only be counting the number of rows that meet this condition.
use below code
$qry=SHOW TABLES FROM 'database_name';
$res=mysql_query($qry);
$output=array();
$i=0;
while($row=mysql_fetch_array($res,MYSQL_NUM)){
++$i;
$sql=SELECT COUNT(*) FROM $row[0];
$output[$i]=mysql_query($sql);
}
$totalRows=array_sum($ouptput);
echo $totalRows;
http://php.net/manual/en/function.mysql-num-rows.php You need this i think.
If you are going to use the following SQL statement:
SELECT COUNT(*) FROM tablename WHERE link='1';
Make sure you have an index on the 'link' column