Pivot Table Using MySQL - mysql

Question

I have two tables Triples and Tags
Triples Table has the following Columns
id PostID TagID Value
1 1 1 Murder
2 1 2 New Brunswick
3 2 1 Theft
4 2 3 Gun
Tags Table has the following Columns
id TagName
1 Incident
2 Location
3 Weapon
I am trying to write sql to create a Pivot Table with Dynamic Headers
Output should be like this
PostID Incident Location Weapon
1 Murder New Brunswick
2 Theft Gun
Any help in writing the SQL would be appreciated. I have seen examples online but could not figure out this one

Answer 1

In order to pivot the data in MySQL, you will need to use both an aggregate function and a CASE expression.
If you have a known number of columns, then you can hard-code the query:
select p.postid,
max(case when t.tagname = 'Incident' then p.value end) Incident,
max(case when t.tagname = 'Location' then p.value end) Location,
max(case when t.tagname = 'Weapon' then p.value end) Weapon
from triples p
left join tags t
on p.tagid = t.id
group by p.postid;
See SQL Fiddle with Demo
But if you have an unknown number of columns, then you will need to use a prepared statement to generate dynamic SQL:
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('SELECT p.postid, ', #sql, '
from triples p
left join tags t
on p.tagid = t.id
group by p.postid');
PREPARE stmt FROM #sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
See SQL Fiddle with Demo.
Both will give the result:
| POSTID | INCIDENT | LOCATION | WEAPON |
----------------------------------------------
| 1 | Murder | New Brunswick | (null) |
| 2 | Theft | (null) | Gun |