使用php连接两个表

Question

I have no idea with joins and I am really having a trouble getting the logic. Can anyone please help me?

Here is my table Announcements:

AnnouncementID      Subject        Header      Status
---------------------------------------------------
1                    Peter        Header 2     Publish
2                     2x2         Header 3     Draft
3                 Resignation     Header 4     Publish

And here is another table ReadAnnouncements:

AnnouncementID      Username      Status
---------------------------------------------
1                    User 1        Read
2                    User 2        Read
2                    User 3        Read

I want my result to be

AnnouncementID      Username      Status    Header       Subject
---------------------------------------------------------------
1                    User 1        Read     Peter        Header 2
2                    User 2        Read     2x2          Header 3
2                    User 3        Read     2x2          Header 3

Please teach me how I am really confused been trying this for two days already.

            <?php 
               $sql=" SELECT a.AnnouncementID,a.Created,r.Username,a.Status,a.Header,a.Body from Announcements a join ReadAnnouncements r using(AnnouncementID) WHERE a.Status = 'Publish'";
               $result = mysqli_query( $conn,$sql);

                 while($rows = mysqli_fetch_array($result)){
                  $time = date('h:i:s a',strtotime($rows['Created']));
                  $date = date('Y-m-d',strtotime($rows['Created']));
                    if($rows['ReadStatus'] == 'Unread'){
                    echo '
                    <tr class="'.$rows['Status'].'clickable-row" >
                      <strong><td class="view-message  dont-show"><a href="ViewAnnouncement.php?view_id='.$rows['AnnouncementID'].'" style="text-decoration: none " class="text-dark"><div>'.$rows['Header'].'</div></a></td>
                      <td class="view-message "><a href="ViewAnnouncement.php?view_id='.$rows['AnnouncementID'].'" style="text-decoration: none" class="text-dark" ><div>'.substr($rows['Body'],0,90).'</div></a></td>
                      <!--<td class="view-message  inbox-small-cells"><i class="fa fa-paperclip"></i></td>-->
                      <td class="view-message  text-right"><a href="ViewAnnouncement.php?view_id='.$rows['AnnouncementID'].'" style="text-decoration: none" class="text-dark"><div><h6>'.$time.''.'<br>'.''.$date.'</h6></div></a></td></strong></tr>                                        
                    ';                                
                    }else{
                       echo '<strong>
                    <tr class="'.$rows['Status'].'clickable-row" >
                      <strong><td class="view-message  dont-show"><a href="ViewAnnouncement.php?view_id='.$rows['AnnouncementID'].'" style="text-decoration: none " class="text-dark"><div>'.$rows['Header'].'</div></a></td>
                      <td class="view-message "><a href="ViewAnnouncement.php?view_id='.$rows['AnnouncementID'].'" style="text-decoration: none" class="text-dark" ><div>'.substr($rows['Body'],0,90).'</div></a></td>
                      <!--<td class="view-message  inbox-small-cells"><i class="fa fa-paperclip"></i></td>-->
                      <td class="view-message  text-right"><a href="ViewAnnouncement.php?view_id='.$rows['AnnouncementID'].'" style="text-decoration: none" class="text-dark"><div><h6>'.$time.''.'<br>'.''.$date.'</h6></div></a></td></strong></tr>                                        
                    </strong>';                             
                    }

                 } 
              ?>

I want to select all rows from table announcements that are only Published and classify them if they are read or unread based on username and announcement id.

Answer 1

You can use the below query to get the result.

select a.AnnouncementID,r.Username,r.Status,a.Header,a.Subject
from Announcements a
join ReadAnnouncements r on r.AnnouncementID=a.AnnouncementID

Answer 2

Joins are pretty easy, check this explanation . In your case, you can do something like this:

SELECT A.AnnouncementID, A.Username, R.Status, A.Header, A.Subject FROM Announcements A join ReadAnnouncements R USING(AnnouncementID)

Answer 3

You can use inner join. The INNER JOIN keyword selects records that have matching values in both tables.

 SELECT a.AnnouncementID,r.Username,r.Status,a.Header,a.Subject
    from Announcements a
    join ReadAnnouncements r using(AnnouncementID)

Answer 4

Use JOIN clause to combine rows from two or more tables in a database, based on a related column between them ( in your case, AnnouncementID ).

When combine data from 2 tables, you have a few combinations possible:

(INNER) JOIN: Returns records that have matching values in both tables LEFT (OUTER) JOIN: Return all records from the left table, and the matched records from the right table RIGHT (OUTER) JOIN: Return all records from the right table, and the matched records from the left table FULL (OUTER) JOIN: Return all records when there is a match in either left or right table (source: https://www.w3schools.com/sql/sql_join.asp )

Using your database schema, you should use:

select * from Announcements as A INNER JOIN ReadAnnouncements as RA ON A.AnnouncementID RA.AnnouncementID

You don't mention which DBMS you are using, so SQL query above may differ.