合并多个SQL查询的结果（由于列名不同，因此无法使用UNION）

Question

I want to make a notifications page which shows notifications about a variety of things, like new followers, new likes, new comments etc. I want to display a list that shows all of these things in chronological order.

My tables look like this:

COMMENT
1   comment__id
2   comment__user_id
3   comment__snap__id
4   comment__text
5   comment_add_time

LIKE
1   like__id
2   like__user__id
3   like__snap__id
4   like__like_time

FOLLOW
1   follow__id
2   follower__user__id
3   followed__user__id
4   follow__follow_time
5   follow__request_status

I would load the followers of a user with a query like this:

try {
    $select_followers_query = '
    SELECT follow.follower__user__id, follow.followed__user__id, follow.follow__request_status, user.user__id, user.user__username, user.user__profile_picture, user.privacy
    FROM follow
    JOIN user ON(follow.follower__user__id = user.user__id) 
    WHERE followed__user__id = :followed__user__id';
    $prep_select_followers = $conn->prepare($select_followers_query);
    $prep_select_followers->bindParam(':followed__user__id', $get_user__id, PDO::PARAM_INT);
    $prep_select_followers->execute();
    $followers_result = $prep_select_followers->fetchAll();
    $followers_count = count($followers_result);
}   
catch(PDOException $e) {
    $conn = null;
    echo $error;
}

Next, I get the results like this:

foreach($followers_result AS $followers_row) {
    $follower_user_id = $followers_row['follower__user__id'];
    // the rest of the variables will come here...
}

I will have separate SQL queries like the one above which each load something. The example above loads the followers, another query will load the comments etc. I want to display the results of all of these queries and display them in chronological order, like this:

@user_1 liked your photo
@user_4 started following you
@user_2 commented on your photo
etc...

How can I achieve this? SQL UNION requires the tables to have the same number of columns and the selected columns must have the same name. I don't have all that. Moreover, every kind of result (follower, comment or like) will have different markups. A follower notification will have a follow button, a comment notification will have a button that redirects to the photo that was liked etc.

Answer 1

You can use UNION but you'll need to use 'AS' to give columns the same name. You'll also need to add a line like this to each select:

, 'comment' as Type FROM comment

and:

, 'follow' as Type FROM follow

Answer 2

SQL UNION requires the tables to have the same number of columns and the selected columns must have the same name.

No, it doesn't. Here table "a" has two columns, integer and varchar.

create table a (
  a_id integer,
  a_desc varchar(10)
);
insert into a values (1, 'aaaaaaaa'), (2, 'bbbbbbbb');

Table "b" has three columns, varchar, date, and char.

create table b (
  b_id varchar(10),
  created_date date,
  unused char(1)
);

insert into b values ('xyz', '2014-01-01', 'x'), ('tuv', '2014-01-13', 'x');

The SQL union operator only requires that the SELECT clauses (not tables) have the same number of columns, and that they be of compatible data types. You can usually cast incompatible types to something more useful.

-- SELECT clause has three columns, but table "a" has only two.
-- The cast is for illustration; MySQL can union an integer with a 
-- varchar without a cast.
--
select cast(a_id as char) as col_1, a_desc as col_2, null as col_3
from a
union all
-- Note that these columns don't have the same names as the columns
-- above.
select b_id, null, created_date 
from b;

You can use a single column for date and varchar, but it's usually not a good idea. (Mixing dates with something that's clearly not a date is usually not a good idea.)

select cast(a_id as char) as col_1, a_desc as col_2
from a
union all
select b_id, created_date 
from b;