SQL select DISTINCT values from multiple columns - mysql

Question

I hope that I formulated tittle right.
I'm trying to make tag's for posts, like for example in instagram.
User can add up to 3 tags. I save them in each in separate database column. (tag1/tag2/tag3) and later want to display only distinct values and their count total. No matter in what column they are located.
For example I have 2 different mysql rows(posts)
row 1 have : tag1 = house, tag2 = kitchen, tag3 = null
row 2 have : tag1 = home, tag2 = garden, tag3 = house
And I want to display house(2)/kitchen(1)/garden(1)/home(1)
Result that I get : house(1)/kitchen(1)/garden(1)/home(1)/house(1) because each house are in different column.
I have database table (diy_posts):
Image
My idea of sql query:
SELECT DISTINCT p.tag1 as tag1, p.tag2 as tag2, p.tag3 as tag3,
SUM(CASE WHEN p.tag1=tag1 OR p.tag2=tag2 OR p.tag3=tag3 THEN 1 ELSE 0 END) as count FROM diy_posts p GROUP BY p.id
And displaying them like:
foreach ($diyTags as $tag) {
echo $tag['tag1']; echo $tag['count'];
echo $tag['tag2']; echo $tag['count'];
echo $tag['tag3']; echo $tag['count'];
}

Answer 1

You can do this by unpivoting the table:
select tag, count(*)
from ((select p.tag1 as tag from diy_posts p) union all
(select p.tag2 as tag from diy_posts p) union all
(select p.tag3 as tag from diy_posts p)
) pt
group by tag;
The need to do this suggests that you may not have the right data model. The more typical model would be:
create table postTags (
postTagid int auto_increment primary key,
postId int,
tag_number int,
tag varchar(255)
);
I do note that this will require a trigger to limit the number of tags to three -- if that is, indeed, desirable. This also makes it possible for the database to prevent duplicate tags, simply by defining a unique constraint or index.

Answer 2

Your Desired Output is not possible with Distinct Keyword.
Also for desired output your Table schema is wrong. it must be verticle instead of horizontal.
Still in the existing Table structure you can get desired output by doing comparision and counting process in ServerSide (PHP seems in your case).
Thats the only way in my guess to achive ouput in existing schema.
- However that will ruin performance when tag and post size increase hence you need to change your table schema or you can approach for the child table too.

Answer 3

Turn it into a single column and do the sum:
select tag, count(*) as counts
from
(
SELECT p.tag1 as tag FROM diy_posts
UNION ALL
SELECT p.tag2 as tag FROM diy_posts
UNION ALL
SELECT p.tag3 as tag FROM diy_posts
) tmp
GROUP BY tag;