如何以一对多关系联接两个表并获得COUNT（）？

Question

I am having a bit of problem with a mysql query that I am developing for a social media application in which users can post their feed/post and view number of "likes", "comments" and "shared" counts -- similar to facebook and twitter.

The part that I am having trouble with is querying feeds and getting the number of "likes" in each feed.

Basically I have two tables:

1. user_feeds is the table that holds the "feeds" data.

2. user_feed_likes holds "likes" of feeds assigned to a feed_id which relates to the id of the user_feeds table

How can I query the feeds and perform an inner subquery to query the count()/ number of likes in the likes table and return the result of the two queries?

The tables look like:

user_feeds

| id | feed       | phone        | time_stamp |
-----------------------------------------------
| 0  | good day   | 25470159224  | 08/08/2018 |
| 1  | bad day    | 254701594224 | 08/09/2018 |
| 2  | normal day | 254701594224 | 08/10/2018 |

user_feed_likes

| id | feed_id | time_stamp |
----------------------------- 
| 0  | 1       | 08/08/2018 |  
| 1  | 1       | 08/08/2018 |  
| 2  | 0       | 09/10/2018 |

I want to select the from user_feeds and get the number of likes from user_feed_likes where user_feed id = [value which is also contained in likes as feed_id]

Here is my php coding attempt that didn't work as desired.

function get_post_id($con,$contact_owned,$shared_post_id,$from,$to){
    $sql="SELECT user_feeds.*,user_credentials.username,profile_pic.imagenamesized,user_feed_likes.user_feed_id AS no_likes

    FROM user_feeds 
    INNER JOIN user_credentials ON user_feeds.phone=user_credentials.phone
    INNER JOIN profile_pic ON user_feeds.phone=profile_pic.phone

    LEFT JOIN user_feed_likes ON user_feed_likes.user_feed_id
    =(SELECT count(user_feed_id)  FROM user_feed_likes WHERE user_feed_id='user_feeds.id')

    WHERE user_feeds.phone IN('".implode("','",$contact_owned)."') ORDER BY time_stamp desc  LIMIT $from,$to;";

    $result=mysqli_query($con,$sql);
    if (mysqli_num_rows($result)>0) {
        while ($rows=$result->fetch_assoc()) {
            $row_arry['id']=$rows['id'];
            $row_arry['phone']=$rows['phone'];
            $row_arry['feed']=$rows['feed'];
            $row_arry['image_1']=$rows['image_1'];
            $row_arry['image_2']=$rows['image_2'];
            $row_arry['Image_3']=$rows['Image_3'];
            $row_arry['image_4']=$rows['image_4'];
            $row_arry['time_stamp']=$rows['time_stamp'];
            $row_arry['username']=$rows['username'];
            $row_arry['imagenamesized']=$rows['imagenamesized'];
            $row_arry['no_likes']=$rows['no_likes'];

            //array_push($respond, $row_arry);

            # code...
        }
        # code...
    }

Any help of how to select a feed and get the number of likes that a feed got will be of great help.

Answer 1

I really, really don't like posting answers that I haven't tested, but for the sake of progressing this question I'll take a stab.

"SELECT
  a.id, a.feed, a.phone, a.time_stamp,
  b.username,
  c.imagenamesized,
  COUNT(d.id) AS likes
FROM user_feeds a
INNER JOIN user_credentials b ON a.phone = b.phone
INNER JOIN profile_pic c ON a.phone = c.phone
LEFT JOIN user_feed_likes d ON a.id = d.feed_id
WHERE a.phone IN ('" . implode("',', $contact_owned) . "')
GROUP BY a.id
ORDER BY a.time_stamp DESC
LIMIT $offset, $limit"

Because it is possible for user_feed_likes to contain zero "likes" for a particular feed_id , a LEFT JOIN is the correct JOIN . Your LEFT JOIN syntax should mirror your INNER JOIN s.

By using GROUP BY you generate aggregate data that COUNT() can happily work with.

To make your query shorter and potentially easier to read, I recommend using table aliases ( a , b , c , d ). When declaring the aliases, you can write the AS keyword before the alias as syntactic sugar, but I think it is more common to omit it.

Your $from and $to are poor choices for variable names because that is not literally the data that they hold. $from isn't terrible, but $to is potentially misleading to future readers of your code. This is why I changed it in my snippet.

As a matter of best practice, you should always write mysql keywords in all caps for readability.

As a matter of best practice, you should avoid using * in your SELECT clause. Only extract the data from columns that you intend to use. This will allow you to loop the result set and simply push the full $row into $respond without explicitly naming each column by [key] .

You should comb over your script again and ensure that you are consistently using object-oriented mysqli syntax. Mixing procedural and object-oriented is not best practice. I prefer object-oriented because it is more succinct.

In case you don't know, $result can be iterated directly with foreach() without calling fetch_assoc() .

Avoid declaring single-use variables. If you are merely pushing the data into another array, do so without intermediate variables.

I don't know if your $contact_owned array (consider renaming this variable as well to better describe the data that it contains. Eg $user_phone_numbers ) is coming from a user submission, but if it is coming from a form submission or other untrusted source, then you MUST implement a prepared statement for security and stability. See this post which speaks about building a prepared statement with a variable number of placeholders: https://stackoverflow.com/a/52323556/2943403

If $contact_owned is derived from a prior query on the database, it would be more direct to reduce the total trips to the database and just write WHERE logic to filter the necessary user phone numbers using a user id or something instead of your current IN logic. This may even allow you to avoid the convoluted prepared statement syntax.