Retrieving data from two tables of mysql - php

Question

I Have Two Tables
Table1
HTNO SUBJECTCODE INTERNALS EXTERNALS TOTAL
1 s1 20 58 78
1 s2 15 20 35
1 s3 10 60 70
2 s1 10 20 30
2 s2 12 30 42
2 s3 15 55 70
.
.
.
so on up to N
Table 2
SUBJECTCODE SUBJECT NAME
s1 MATHS
s2 SCIENCE
s3 SOCIAL
I will be giving a form for student to enter the hallticket Number
If student Enters 1 in form then the result should be
Subjectcode SubjectName Internals Externals Total
s1 Maths 20 58 78
s2 Science 15 20 35
s3 Social 10 60 70
The above should be the output
But Here I am unable to retrieve SubjectName from Table2 in the result
And here is my code which i am using
<?PHP
$userInputEntities = htmlentities($userInput);
echo $userInputEntities;
$username = "admin";
$password = "123456";
$database = "test";
$server = "localhost";
$db = new PDO ("mysql:host=$server;dbname=$database", "$username", "$password");
if ($db) {
$id = $_GET['id'];
$SQL = $db->prepare("SELECT * FROM Table1 WHERE htno = :id");
$SQL -> execute(array(':id'=>$id));
$n = $SQL->rowCount();
echo "
<center><table class='dynamic styled with-prev-next' data-table-tools='{'display':true}' align=center>
<thead>
<tr>
<TH class='table-header dark' scope='col'>SUBJECT CODE</TH>
<TH class='table-header dark' scope='col'>SUBJECT NAME</TH>
<TH class='table-header dark' scope='col'>INTERNALS</TH>
<TH class='table-header dark' scope='col'>EXTERNALS</TH>
<TH class='table-header dark' scope='col'>TOTAL</TH>
</tr></thead><center>";
while ($db_field = $SQL->fetch(PDO::FETCH_ASSOC)) {
echo "<tr><tbody>";
echo "<td align=center>" . $db_field['SubjectCode'] . "</td>";
echo "<td align=center>" . $db_field['Internals'] . "</td>";
echo "<td align=center>" . $db_field['Externals'] . "</td>";
echo "<td align=center>" . $db_field['Total'] . "</td>";
echo "</tbody></tr>";
}
with this code i am unable to get SUbject Name for a particualr subject code of a student
actually i have nt written any code to retrieve dubject name from Table2, I dont Know How to write it
Please Help me

Answer 1

Try this:
$SQL = $db->prepare(
"SELECT T2.Subjectcode, T2.SubjectName, T1.Internals, T1.Externals, T1.Total FROM Table1 as T1
JOIN Table2 as T2
ON T1.SUBJECTCODE = T2.SUBJECTCODE
WHERE T1.HTNO = :id");

Answer 2

You would need to do a join in your query.
MySQL documentation : Join

Answer 3

It looks like you need a SQL statement that will retrieve the resultset you want:
SELECT t1.SubjectCode
, t2.SubjectName
, t1.Internals
, t1.Externals
, t1.Total
FROM Table1 t1
JOIN Table2 t2
ON t2.SubjectCode = t1.SubjectCode
WHERE t1.htno = :id
To have the rows returned in a predictable order, you can include an ORDER BY clause in the query text, following the WHERE clause:
ORDER
BY t1.htno
, t1.SubjectCode
Q: "Before the result was faster but now it is taking much time to display the result - can u tell me y if u know the reason ?"
A: No, I don't have enough information to determine the exact reason for the slow performance of the new statement. But I can give you some likely possibilities.
(I have to tell you though, that I am hesitant to respond to your query, since you have already "selected" an answer to your question.)
The most likely explanation for the slow performance is that you do not have appropriate indexes defined on your tables. And the most likely candidate indexes (for best performance of your query) would be:
... Table2_IX1 ON Table2 (SubjectCode, SubjectName)
and
... Table1_IX1 ON Table1 (htno, SubjectCode, Internals, Externals, Total)
On Table, at a minimum, you want an index that has a leading column of htno, since your query includes an equality predicate (i.e. WHERE htno = 'literal constant'.
And it would be beneficial to have the next column in that same index be SubjectCode, especially if you specify ORDER BY t1.SubjectCode (or ORDER BY t1.htno, t1.SubjectCode) in your query, since MySQL can make use of that index, and bypass a "filesort" operation that would otherwise be required.
If you also include all of the other columns from Table1 that are referenced by your query, then you would have a "covering" index. That means that MySQL can obtain all of the data in needs directly from the index pages, without having to visit the pages of the underlying table.
On Table2, at a minimum, you want an index with a leading column of SubjectCode. That will allow MySQL to use that index to satisfy the join predicate. If that same index also includes theSubjectName` column, then that index would also be a "covering" index for your query, and MySQL could satisfy the query entirely from the index, without a need to visit any pages in the underlying table.
To really evaluate which indexes will give the best performance, you'd need to EXPLAIN your query, and take a look at the access paths. It's likely that the best performance of this query will be obtained when the Extra column in the EXPLAIN output shows "Using index".