FeeJai FeeJai - 1 year ago 940
SQL Question

MySQL pivot row into dynamic number of columns

Lets say i have three different mysql tables:



id | name
1 Product A
2 Product B


id | name
1 Partner A
2 Partner B


partners_id | products_id
1 2
2 5
1 5
1 3
1 4
1 5
2 2
2 4
2 3
1 1

I would like to get a table with partners in the rows and products as columns. So far I was able to get an output like this:

name | name | COUNT( * )
Partner A Product A 1
Partner A Product B 1
Partner A Product C 1
Partner A Product D 1
Partner A Product E 2
Partner B Product B 1
Partner B Product C 1
Partner B Product D 1
Partner B Product E 1

using this query:

SELECT partners.name, products.name, COUNT( * )
FROM sales
JOIN products ON sales.products_id = products.id
JOIN partners ON sales.partners_id = partners.id
GROUP BY sales.partners_id, sales.products_id
LIMIT 0 , 30

but I would like to have instead something like:

partner_name | Product A | Product B | Product C | Product D | Product E
Partner A 1 1 1 1 2
Partner B 0 1 1 1 1

The problem is that I cannot tell how many products I will have so the column number needs to change dynamically depending on the rows in the products table.
This very good answer does not seem to work with mysql: T-SQL Pivot? Possibility of creating table columns from row values

Answer Source

Unfortunately MySQL does not have a PIVOT function which is basically what you are trying to do. So you will need to use an aggregate function with a CASE statement:

select pt.partner_name,
  count(case when pd.product_name = 'Product A' THEN 1 END) ProductA,
  count(case when pd.product_name = 'Product B' THEN 1 END) ProductB,
  count(case when pd.product_name = 'Product C' THEN 1 END) ProductC,
  count(case when pd.product_name = 'Product D' THEN 1 END) ProductD,
  count(case when pd.product_name = 'Product E' THEN 1 END) ProductE
from partners pt
left join sales s
  on pt.part_id = s.partner_id
left join products pd
  on s.product_id = pd.prod_id
group by pt.partner_name

See SQL Fiddle with Demo

Since you do not know the Products you will probably want to perform this dynamically. This can be done using prepared statements.

Dynamic pivot tables (transform rows to columns)

Your code would look like this:

SET @sql = NULL;
      'count(case when Product_Name = ''',
      ''' then 1 end) AS ',
      replace(Product_Name, ' ', '')
  ) INTO @sql
from products;

SET @sql = CONCAT('SELECT pt.partner_name, ', @sql, ' from partners pt
left join sales s
  on pt.part_id = s.partner_id
left join products pd
  on s.product_id = pd.prod_id
group by pt.partner_name');

PREPARE stmt FROM @sql;

See SQL Fiddle with Demo

It's probably worth noting that while @bluefeet's solution is excellent, GROUP_CONCAT is by default limited to 1024 bytes. You can work around this by setting it higher for the duration of your procedure, ie. SET @@group_concat_max_len = 32000;