SELECT * FROM table_1 WHERE ID =表_2中的ID

Question

Been working on this for a while; Need to select data from a table where the ID column matches the ID from another table.

My code so far:

<?php
$sql = "SELECT * FROM input";
$sql2 = "SELECT * FROM output WHERE question_id =".$row["id"];
$result = $conn->query($sql);
$result2 = $conn->query($sql2);

if ($result->num_rows > 0) {
  $index = 0;
  while($row = $result->fetch_assoc()) {
    $index++;
    ?>

    <?php
    echo '<input type="hidden" name="questionid" value="'. $row['id'].'"/>';
    ?>

    <?php

    if ($result2->num_rows > 0) {
      while($row2 = $result2->fetch_assoc()) {
      }
    } else {
      echo "0 results";
    }
  }
} else {
  echo "0 results";
}
$conn->close();
?>

I know I m missing something important here. Any suggestions are appreciated.

Answer 1

In order to achieve this thing you can use the JOIN query which is available in mysql.

Ex: Consider two tables user and course

User Table:

id  name         course
1   Alice        1
2   Bob          1
3   Caroline     2
4   David        5
5   Emma        (NULL)

Course Table:

id  name         
1   HTML5
2   CSS3
3   JS
4   PHP
5   WORDPRESS

INNER JOIN (or just JOIN)

The most frequently used clause is INNER JOIN. This produces a set of records which match in both the user and course tables, ie all users who are enrolled on a course:

SELECT user.name, course.name FROM `user` INNER JOIN `course` on user.course = course.id;

LEFT JOIN

What if we require a list of all students and their courses even if they're not enrolled on one? A LEFT JOIN produces a set of records which matches every entry in the left table (user) regardless of any matching entry in the right table (course):

SELECT user.name, course.name FROM `user` LEFT JOIN `course` on user.course = course.id;

RIGHT JOIN

Perhaps we require a list all courses and students even if no one has been enrolled? A RIGHT JOIN produces a set of records which matches every entry in the right table (course) regardless of any matching entry in the left table (user):

SELECT user.name, course.name FROM `user` RIGHT JOIN `course` on user.course = course.id;

Answer 2

You missed fetching the row items to extract the ID you want to use as CONSTRAINT in the next query

$sql = "SELECT * FROM input";

$row = mysqli_fetch_array($sql);

    $sql2 = "SELECT * FROM output WHERE question_id =".$row["id"];
    $result = $conn->query($sql);
    $result2 = $conn->query($sql2);

You can also consider JOIN SELECT of the two tables like this;

$sql = SELECT input.* , output.* FROM input JOIN output (id) WHERE input.id = "".

check out MySql Join three tables

Answer 3

不确定这是否是您想要的，因此在拒绝投票之前在评论中让我知道，请尝试以下操作：

SELECT * FROM output, input WHERE output.question_id = input.id

Answer 4

You can achieve this using SQL INNER JOIN Keyword

SQL

SELECT * FROM input INNER JOIN output on input.id = output.question_id

Answer 5

Use this inner join to get row from both table where the ids are match

SELECT *
FROM input
INNER JOIN output
ON input.id=output.question_id;

Answer 6

You should rather join both tables with an INNER JOIN , which will discard rows from tables if the id is not present in the other table. Try something like:

SELECT * FROM tbl1 INNER JOIN tbl2 ON tbl1.id = tbl2.id;