I am not a professional programmer, but I assist a school in automating their assessments. I have a list of just over 1000 students with 3 assessment scores for each one every year and I need to create a list with the average of these three scores in descending order, limiting it to the top 30. I can calculate averages and display the results, but I can't sort or limit. In the first part of the code, I select the IDs from all students and store them into the array $alunos[] for the current year ($IDanoatual). In the second part, I use a for loop to calculate the average of these grades for each student and display them. Both codes lookup the same table ( audp_l_notasfinais). I tried using the foreach statement to filter and sort, but I couldn't resolve this issue.
$sela = "select id_aluno from audp_l_notasfinais
where id_ano = '$IDanoatual'
";
$qsela = mysqli_query($conn,$sela);
$contasel = mysqli_num_rows($qsela);
while ($row = mysqli_fetch_assoc($qsela)){
$alunos[] = $row['id_aluno'];
}
for ($i=0; $i<$contasel; $i++){
$selnotasA = "select avg(NULLIF(nts.mef,0)) as NtutA, aln.stdname Naln
from audp_l_notasfinais nts
inner join audp_c_alunos aln on aln.id_alunos = nts.id_aluno
where nts.id_aluno = '$alunos[$i]' and nts.id_ano='$IDanoatual'
";
$qrynmal = mysqli_query($conn,$selnotasA);
while ($row=mysqli_fetch_assoc($qrynmal)){
echo "Name: ".$row['Naln']." - Average: ".$row['NtutA']."<br>";
}
}
You have not included much detail in your question. Adding your CREATE TABLE statements and some sample data in markdown tables would help, and get a better response.
It looks like $IDanoatual could be coming from user input, in which case you really need to read about and understand SQL Injection and how to mitigate the risk with prepared statements.
Best guess -
select aln.id_alunos, aln.stdname Naln, avg(NULLIF(nts.mef,0)) as NtutA
from audp_c_alunos aln
inner join audp_l_notasfinais nts
on aln.id_alunos = nts.id_aluno
and nts.id_ano = '$IDanoatual'
group by aln.id_alunos
order by NtutA desc
limit 30;
Related
I have a query below that produces a simple list of plants (the field named "thriller")
Problem:
Query below considers only the first sequential set of matching items from the table, then displays them in random order. The query is not considering other matching items that are found later in the table.
Solution Needed:
I want the query to choose random results from ALL items in the table, not only the first set of matching results.
$row_object = $wpdb->get_results( $wpdb->prepare(
"SELECT * FROM {$wpdb->prefix}personal_creations_assistant
WHERE pot_sun_exposure = %s
AND pot_height = %s
AND pot_size = %s
AND pot_shape = %s
AND pot_placement = %s
GROUP BY thriller
ORDER BY RAND() LIMIT 0,3
",
$sun_exposure,
$height,
$size,
$shape,
$placement
)
);
If i get it right, you want to get some random rows from database. Random 3 rows.
You could do it with php. First make a COUNT to find how many rows you have total. Then say you call your variable $rowstotal; you make another one $limit_s = rand(0, $rowstotal); Then you add it to your query:
... LIMIT $limit_s, 3
Hi I need to get the results and apply the order by only in the limited section. You know, when you apply order by you are ordering all the rows, what I want is to sort only the limited section, here is an example:
// all rows
SELECT * FROM users ORDER BY name
// partial 40 rows ordered "globally"
SELECT * FROM users ORDER BY name LIMIT 200,40
The solution is:
// partial 40 rows ordered "locally"
SELECT * FROM (SELECT * FROM users LIMIT 200,40) AS T ORDER BY name
This solution works well but there is a problem: I'm working with a Listview component that needs the TOTAL rows count in the table (using SQL_CALC_FOUND_ROWS). If I use this solution I cannot get this total count, I will get the limited section count (40).
I hope you will give me solution based on the query, for example something like: "ORDER BY LOCALLY"
Since you're using PHP, might as well make things simple, right? It is possible to do this in MySQL only, but why complicate things? (Also, placing less load on the MySQL server is always a good idea)
$result = db_query_function("SELECT SQL_CALC_FOUND_ROWS * FROM `users` LIMIT 200,40");
$users = array();
while($row = db_fetch_function($result)) $users[] = $row;
usort($users,function($a,$b) {return strnatcasecmp($a['name'],$b['name']);});
$totalcount = db_fetch_function(db_query_function("SELECT FOUND_ROWS() AS `count`"));
$totalcount = $totalcount['count'];
Note that I used made-up function names, to show that this is library-agnostic ;) Sub in your chosen functions.
I have a database representing something like a bookstore. There's a table containing the categories that books can be in. Some categories are defined simply using another table that contains the category-item relationships. But there are also some categories that can be defined programmatically -- a category for a specific author can be defined using a query (SELECT item_id FROM items WHERE author = "John Smith"). So my categories table has a "query" column; if it's not null, I use this to get the items in the category, otherwise I use the category_items table.
Currently, I have the application (PHP code) make this decision, but this means lots of separate queries when we iterate over all the categories. Is there some way to incorporate this dynamic SQL into a join? Something like:
SELECT c.category, IF(c.query IS NULL, count(i.items), count(EXECUTE c.query)
FROM categories c
LEFT OUTER JOIN category_items i
ON c.category = i.category
EXECUTE requires a prepared statement, but I need to prepare a different statement for each row. Also, EXECUTE can't be used in expressions, it's just a toplevel statement. Suggestions?
What happens when you want to list books by publisher? Country? Language? You'd have to throw them all into a single "category_items" table. How would you pick which dynamic query to execute? The query-within-a-query method is not going to work.
I think your concept of "category" is too broad, which is resulting in overly complicated SQL. I would replace "category" to represent only "genre" (for books). Genres are defined in their own table, and item_genres connects them to the items table. Books-by-author and books-by-genre should just be separate queries at the application level, rather than trying to do them both with the same (sort of) query at the database/SQL level. (If you have music as well as books, they probably shouldn't all be stored in a single "items" table because they're different concepts ... have different genres, author vs. artist, etc.)
I know this does not really solve your problem in the way you'd like, but I think you'll be happier not trying to do it that way.
Here's how I finally ended up solving this in the PHP client.
I decided to just keep the membership in the category_items table, and use the dynamic queries during submission to update this table.
This is the function in my script that's called to update an item's categories during submission or updating. It takes a list of user-selected categories (which can only be chosen from categories that don't have dynamic queries), and using this and the dynamic queries it figures out the difference between the categories that an item is currently in and the ones it should be in, and inserts/deletes as necessary to get them in sync. (Note that the actual table names in my DB are not the same as in my question, I was using somewhat generic terms.)
function update_item_categories($dbh, $id, $requested_cats) {
$data = mysql_check($dbh, mysqli_query($dbh, "select id, query from t_ld_categories where query is not null"), 'getting dynamic categories');
$clauses = array();
while ($row = mysqli_fetch_object($data))
$clauses[] = sprintf('select %d cat_id, (%d in (%s)) should_be_in',
$row->id, $id, $row->query);
if (!$requested_cats) $requested_cats[] = -1; // Dummy entry that never matches cat_id
$requested_cat_string = implode(', ', $requested_cats);
$clauses[] = "select c.id cat_id, (c.id in ($requested_cat_string)) should_be_in
from t_ld_categories c
where member_type = 'lessons' and query is null";
$subquery = implode("\nunion all\n", $clauses);
$query = "select c.cat_id cat_id, should_be_in, (member_id is not null) is_in
from ($subquery) c
left outer join t_ld_cat_members m
on c.cat_id = m.cat_id
and m.member_id = $id";
// printf("<pre>$query</pre>");
$data = mysql_check($dbh, mysqli_query($dbh, $query), 'getting current category membership');
$adds = array();
$deletes = array();
while ($row = mysqli_fetch_object($data)) {
if ($row->should_be_in && !$row->is_in) $adds[] = "({$row->cat_id}, $id)";
elseif (!$row->should_be_in && $row->is_in) $deletes[] = "(cat_id = {$row->cat_id} and member_id = $id)";
}
if ($deletes) {
$delete_string = implode(' or ', $deletes);
mysql_check($dbh, mysqli_query($dbh, "delete from t_ld_cat_members where $delete_string"), 'deleting old categories');
}
if ($adds) {
$add_string = implode(', ', $adds);
mysql_check($dbh, mysqli_query($dbh, "insert into t_ld_cat_members (cat_id, member_id) values $add_string"),
"adding new categories");
}
}
Good evening guys,
I'm a newbie to web programming and I need your help to solve a problem inherent to SQL query.
The database engine I'm using is MySQL and I access it via PHP, here I'll explain a simplified version of my database, just to fix ideas.
Let's suppose to work with a database containing three tables: teams, teams_information, attributes. More precisely:
1) teams is a table containing some basic information about italian football teams (soccer, not american football :D), it is formed by three fields: 'id' (int, primary key), 'name' (varchar, team name), nickname (Varchar, team nickname);
2) attributes is a table containing a list of possible information about a football team, such as city (the city where team plays its home match), captain (team captain's fullname), f_number (number of fans) and so on. This table is formed by three fields: id (int, primary key), attribute_name (varchar, an identifier for the attribute), attribute_desc (text, an explanation of the meaning of attribute). Each record of this table represents a single possible attribute of a football team;
3) teams_information is a table where some information, about teams listed in team table, are available. This table contains three fields: id (int, primary key), team_id (int, a foreign key which identifies a team), attribute_id (int, a foreign key which identifies one of the attributes listed in attributes table), attribute_value (varchar, the value of the attribute). Each record represents a single attribute of a single team. In general, different teams will have a different number of information, so for some teams a large number of attributes will be available while for other teams only a small number of attributes will be available.
Note that relation between teams and teams_information is one to many and the same relation exists between attributes and teams_information
Well, given this model my purpose is to realize a grid (maybe with ExtJS 4.1) to show user the list of italian football team, each record of this grid will represent a single football team and will contain all possible attributes: some fields may be empty (because, for considered team, the correspondent attribute is unknown), while the others will contain the values stored in teams_information table (for the considered team).
According to the above grid's field are: id, team_name and a number of fields to represent all the different attributes listed in 'attributes' table.
My question is: can I realize such a grid by using a SINGLE SQL query (maybe a proper SELECT query, to fetch all data I need from database tables) ?
Can anyone suggest me how to write a similar query (if it exists) ?
Thanks in advance for helping me.
Regards.
Enrico.
The short answer to your question is no, there is no simple construct in MySQL to achieve the result set you are looking for.
But it is possible to carefully (painstakingly) craft such a query. Here is an example, I trust you will be able to decipher it. Basically, I'm using correlated subqueries in the select list, for each attribute I want returned.
SELECT t.id
, t.name
, t.nickname
, ( SELECT v1.attribute_value
FROM team_information v1
JOIN attributes a1
ON a1.id = v1.attribute_id AND a1.attribute_name = 'city'
WHERE v1.team_id = t.id ORDER BY 1 LIMIT 1
) AS city
, ( SELECT v2.attribute_value
FROM team_information v2 JOIN attributes a2
ON a2.id = v2.attribute_id AND a2.attribute_name = 'captain'
WHERE v2.team_id = t.id ORDER BY 1 LIMIT 1
) AS captain
, ( SELECT v3.attribute_value
FROM team_information v3 JOIN attributes a3
ON a3.id = v3.attribute_id AND a3.attribute_name = 'f_number'
WHERE v3.team_id = t.id ORDER BY 1 LIMIT 1
) AS f_number
FROM teams t
ORDER BY t.id
For 'multi-valued' attributes, you'd have to pull each instance of the attribute separately. (Use the LIMIT to specify whether you are retrieving the first one, the second one, etc.)
, ( SELECT v4.attribute_value
FROM team_information v4 JOIN attributes a4
ON a4.id = v4.attribute_id AND a4.attribute_name = 'nickname'
WHERE v4.team_id = t.id ORDER BY 1 LIMIT 0,1
) AS nickname_1st
, ( SELECT v5.attribute_value
FROM team_information v5 JOIN attributes a5
ON a5.id = v5.attribute_id AND a5.attribute_name = 'nickname'
WHERE v5.team_id = t.id ORDER BY 1 LIMIT 1,1
) AS nickname_2nd
, ( SELECT v6.attribute_value
FROM team_information v6 JOIN attributes a6
ON a6.id = v6.attribute_id AND a6.attribute_name = 'nickname'
WHERE v6.team_id = t.id ORDER BY 1 LIMIT 2,1
) AS nickname_3rd
I use nickname as an example here, because American soccer clubs frequently have more than one nickname, e.g. Chicago Fire Soccer Club has nicknames: 'The Fire', 'La Máquina Roja', 'Men in Red', 'CF97', et al.)
NOT AN ANSWER TO YOUR QUESTION, BUT ...
Have I mentioned numerous times before, how much I dislike working with EAV database implementations? What should IMO be a very simple query turns into an overly complicated beast of a potentially light dimming query.
Wouldn't it be much simpler to create a table where each "attribute" is a separate column? Then queries to return reasonable result sets would look more reasonable...
SELECT id, name, nickname, city, captain, f_number, ... FROM team
But what really makes me shudder is the prospect that some developer is going to decide that the LDQ should be "hidden" in the database as a view, to enable the "simpler" query.
If you go this route, PLEASE PLEASE PLEASE resist any urge you may have to store this query in the database as a view.
I'm going to take a slightly different route. Spencer's answer is fantastic, and it addresses the issue quite well, but there's still a large underlying problem.
The data that you are trying to display on the site is over-normalized in the database. I won't elaborate, since, again, Spencer's answer highlights the issue pretty well.
Rather, I'd like to recommend a solution that denormalizes the data a bit.
Convert all of your Team data into a single table with many columns. (If there is Player data that isn't covered in the question, that would be a second table, but I'll gloss over that for now.)
Sure, you'll have a whole bunch of columns, and a lot of the columns might be NULL for a lot of the rows. It's not normalized, and it's not pretty, but here's the huge advantage that you gain.
Your query becomes:
SELECT * FROM Teams
That's it. That gets displayed right to the website and you are done. You might have to go out of your way to realize this schema, but it would be totally worth the time investment.
I think what you're saying is that you want the rows in the attributes table to appear as columns in the result recordset. If this is correct, then then in SQL you would use PIVOT.
A quick search on SO seems to indicate that there is no PIVOT equivalent in MySql.
I wrote a simple PHP script to generalize spencer's idea to solve my issue.
Here's the code:
<?php
require_once('includes/db.config.php'); //this file performs connection to mysql
/*
* Following function requires a table name ($table)
* and a number of service fields ($num). Given those parameters
* it returns the number of table fields (excluding service fields).
*/
function get_fields_number($table,$num,$conn)
{
$query = "SELECT * FROM $table";
$result = mysql_query($query,$conn);
return mysql_num_fields($result)-$num; //remember there are $num service fields
}
/*
* Following function requires a table name ($table) and an array
* containing a list of service fields names. Given those parameters,
* it returns the list of field names. That list is contained within an array and
* service fields are excluded.
*/
function get_fields_name($table,$service,$conn)
{
$query = "SELECT * FROM $table";
$result = mysql_query($query,$conn);
$name = array(); //Array to be returned
for ($i=0;$i<mysql_num_fields($result);$i++)
{
if(!in_array(mysql_field_name($result,$i),$service))
{
//currently selected field is not a service field
$name[] = mysql_field_name($result,$i);
}
}
return $name;
}
//Below $conn is db connection created in 'db.config.php'
$query = "SELECT `name` FROM `detail_arg` WHERE visibility = 0";
$res = mysql_query($query,$conn);
if($res===false)
{
$err_msg = mysql_real_escape_string(mysql_error($conn));
echo "{success:false,data:'".$err_msg."'}";
die();
}
$arg = array(); //list of argument names
while($row = mysql_fetch_assoc($res))
{
$arg[] = $row['name'];
}
//Following function writes the select subquery which is
//necessary to build a column containing a single attribute.
function make_subquery($attribute) //$attribute contains attribute name
{
$query = "";
$query.="(SELECT incident_detail.arg_value ";
$query.="FROM incident_detail ";
$query.="INNER JOIN detail_arg ";
$query.="ON incident_detail.arg_id = detail_arg.id AND detail_arg.name='".$attribute."' ";
$query.="WHERE incident.id = incident_detail.incident_id) ";
$query.="AS $attribute";
return $query;
}
/*
echo make_subquery("date"); //debug code
*/
$subquery = array(); //list of subqueries
for($i=0;$i<count($arg);$i++)
{
$subquery[] = make_subquery($arg[$i]);
}
$query = "SELECT "; //final query containing subqueries
$fields = get_fields_name("incident",array("id","visibility"),$conn);
//list of 'incident' table's fields
for($i=0;$i<count($fields);$i++)
{
$query.="incident.".$fields[$i].", ";
}
//insert the subqueries
$sub = implode($subquery,", ");
$query .= $sub;
$query.=" FROM incident ORDER BY incident.id";
echo $query;
?>
my $sth = $dbh->prepare("SELECT id
FROM user
WHERE group == '1'
ORDER BY id DESC
LIMIT 1");
I was trying to get the id of the last row in a table without reading the whole table.
I am already accessing via:
my $sth = $dbh->prepare("SELECT name,
group
FROM user
WHERE group == '1'
LIMIT $from, $thismany");
$sth->execute();
while(my ($name,$group) = $sth->fetchrow_array()) {
...and setting up a little pagination query as you can see.
But, I am trying to figure out how to detect when I am on the last (<= 500) rows so I can turn off my "next 500" link. Everything else is working fine. I figured out how to turn off the "previous 500" link when on first 500 page all by myself!
I thought I would set up a "switch" in the while loop so if ($id = $last_id) I can set the "switches" var.
Like:
if ($id = $last_id) {
$lastpage = 1; #the switch
}
So I can turn off next 500 link if ($lastpage == 1).
I am really new to this and keep getting stuck on these types of things.
Thanks for any assistance.
Try to grab an extra row and see how many rows you really got. Something like this:
my #results = ( );
my $total = 0;
my $sth = $dbh->prepare(qq{
SELECT name, group
FROM user
WHERE group = ?
LIMIT ?, ?
});
$sth->execute(1, $from, $thismany + 1);
while(my ($name, $group) = $sth->fetchrow_array()) {
push(#results, [$name, $group]); # Or something more interesting.
++$total;
}
$sth->finish();
my $has_next = 0;
if($total == $thismany + 1) {
pop(#results);
$has_next = 1;
}
And BTW, please use placeholders in all of your SQL, interpolation is fraught with danger.
Always asking for one more row than you are going to show, as suggested by mu is too short, is a good way.
But if you want to take the other suggested approach of doing two separate queries, one to get the desired rows, and one to get the total count if there had not been a limit clause, MySQL provides an easy way to do that while combining as much of the work as possible:
SELECT SQL_CALC_FOUND_ROWS name, group FROM user WHERE group = '1' LIMIT ..., ...;
then:
SELECT FOUND_ROWS();
The SQL_CALC_FOUND_ROWS qualifier changes what a following FOUND_ROWS() returns without requiring you to do a whole separate SELECT COUNT(*) from user WHERE group = '1' query.
SELECT COUNT(*) from tablename will give you the number of rows, so if you keep a running count of how many rows you have read so far, you'll know when you're on the last page of results.
You could generate that query with (untested; away from a good workstation at the moment):
my $sth = $dbh->prepare("select COUNT(*) FROM user WHERE group == '1'");
my #data = $sth->fetchrow_array;
my $count = $data->[0];
(PS. you should be aware of SQL injection issues -- see here for why.)
As Ether mentioned in the comments, pagination usually requires two queries. One to return your paged set, the other to return the total number of records (using a COUNT query).
Knowing the total number of records, your current offset and the number of records in each page is enough data to work out how many pages there are in total and how many before and after the current page.
Although your initial suggestion of SELECT id FROM table WHERE ... ORDER BY id DESC LIMIT 1 should work for finding the highest matching ID, the standard way of doing this is SELECT max(id) FROM table WHERE ...