Pivot Table Using MySQL and Group Conact - mysql

Question

This is a Question related to my original question
Pivot Table Using MySQL
Data in the tables are saved like this now
I have two tables Triples and Tags
Triples Table has the following Columns
id PostID TagID Value
1 1 1 Murder
2 1 1 Theft
3 2 2 Knife
4 2 2 Gun
Tags Table has the following Columns
id TagName
1 Incident
2 Weapon
I am trying to write sql to create a Pivot Table with Dynamic Headers
Output should be like this
PostID Incident Weapon
1 Murder,Theft
2 Knifie,Gun
I written a inefficient and partial sql query for this, Any help in writing this sql appreciated
SET #sql = NULL;
SELECT
GROUP_CONCAT(DISTINCT
CONCAT(
'max(CASE WHEN TagName = ''',
TagName,
''' THEN p.value END) AS `',
TagName, '`'
)
) INTO #sql
FROM tags;
SET #sql
= CONCAT('CREATE VIEW PostTags as SELECT p.postid, p.id, ', #sql, '
from triples p
left join tags t
on p.tagid = t.id
group by p.postid, p.id');
PREPARE stmt FROM #sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

Answer 1

You will have to build two strings, one for the inner rotation to create the columns of Incident and Weapon, and the second string with use GROUP_CONCAT for the outer query:
SET #sql = NULL;
SET #sql2 = NULL;
SELECT
GROUP_CONCAT(DISTINCT
CONCAT(
'group_concat(',
TagName,
' separator '', '') as `',
TagName, '`'
)
)
INTO #sql2
FROM tags;
SELECT
GROUP_CONCAT(DISTINCT
CONCAT(
'max(CASE WHEN TagName = ''',
TagName,
''' THEN p.value END) AS `',
TagName, '`'
)
)
INTO #sql
FROM tags;
SET #sql = CONCAT('select postid, ', #sql2, '
from
(
SELECT p.postid, p.id, ', #sql, '
from triples p
left join tags t
on p.tagid = t.id
group by p.postid, p.id
) d
group by postid');
PREPARE stmt FROM #sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
See SQL Fiddle with Demo
This is creating the SQL code:
select postId,
group_concat(incident separator ', ') incident,
group_concat(Weapon separator ', ') Weapon
from
(
SELECT p.postid, p.id,
max(CASE WHEN TagName = 'Incident' THEN p.value END) AS `Incident`,
max(CASE WHEN TagName = 'Weapon' THEN p.value END) AS `Weapon`
from triples p
left join tags t
on p.tagid = t.id
group by p.postid, p.id
) d
group by postId;
See SQL Fiddle with Demo