Mysql - How to create view representing dynamic pivot - mysql

Question

I have two database tables customers which contains data about customers with the scheme like that:
mysql> SELECT * FROM customers;
customer_id created_at partner_id
1 "2019-08-20 09:17:58" cats
2 "2019-09-12 11:46:37" dogs
and customers_facts which keeps the customers facts in a form of fact_name and corresponding fact_value.
mysql> SELECT * FROM customers_facts;
customer_id fact_name fact_value
1, name Milton
1 city Milan
2 surname Bloom
2 name Orlando
I want to create a pivot table which in each row will have a customer and it's facts each as a separate column. Something like this:
mysql> SELECT * FROM pivot_table;
customer_id created_at partner_id name city surname
1 "2019-08-20 09:17:58" cats Milton Milan
2 "2019-09-12 11:46:37" dogs Orlando Bloom
I've found a script that allows me to create such table:
SET #sql = '';
SELECT
#sql := CONCAT(#sql,if(#sql='','',', '),temp.output)
FROM
(
SELECT
DISTINCT
CONCAT(
'MAX(IF(cf.fact_name = ''',
fact_name,
''', cf.fact_value, NULL)) AS ''',
fact_name,
''''
) as output
FROM
customers_facts
) as temp;
SET #sql = CONCAT('SELECT c.customer_id, c.created_at, c.partner_id, ', #sql, '
FROM customers c
LEFT JOIN customers_facts AS cf
ON cf.customer_id = c.customer_id
GROUP BY c.customer_id, c.created_at, c.partner_id');
but I have an issue of how to make it so:
a) I will be able to query the pivot table
b) When I add a new entry / update an old one in one of those two original tables the pivot table will be updated
How to solve ? Is it possible ?

Answer 1

Consider the following:
DROP TABLE IF EXISTS customers;
CREATE TABLE customers
(customer_id SERIAL PRIMARY KEY
,created_at DATETIME NOT NULL
,partner_id INT NOT NULL
);
INSERT INTO customers VALUES
(1,"2019-08-20 09:17:58",108),
(2,"2019-09-12 11:46:37",110);
DROP TABLE IF EXISTS customers_facts ;
CREATE TABLE customers_facts
(customer_id INT NOT NULL
,fact_name VARCHAR(20) NOT NULL
,fact_value VARCHaR(20) NOT NULL
,PRIMARY KEY(customer_id,fact_name)
);
INSERT INTO customers_facts VALUES
(1,'name','Milton'),
(1,'city','Milan'),
(2,'surname','Bloom'),
(2,'name','Orlando');
Now we can create a VIEW in the manner you describe...
DROP VIEW IF EXISTS my_pivot;
CREATE VIEW my_pivot AS
SELECT c.customer_id
, c.created_at
, c.partner_id
, MAX(CASE WHEN fact_name = 'name' THEN fact_value END) name
, MAX(CASE WHEN fact_name = 'surname' THEN fact_value END) surname
, MAX(CASE WHEN fact_name = 'city' THEN fact_value END) city
FROM customers c
LEFT
JOIN customers_facts f
ON f.customer_id = c.customer_id
GROUP
BY c.customer_id;
We can interrogate this VIEW with a simple query - e.g. SELECT customer_id FROM my_pivot WHERE name = 'Milton', however, this cannot use an index, so it's not very efficient.
Also, because of the way in which we created the VIEW, it cannot be updated...
UPDATE my_pivot SET name = 'Leonardo' WHERE customer_id = 1;
ERROR 1288 (HY000): The target table my_pivot of the UPDATE is not updatable
However, had we created the VIEW slightly differently, then it could be updated...
DROP VIEW IF EXISTS my_pivot;
CREATE VIEW my_pivot AS
SELECT c.customer_id
, c.created_at
, c.partner_id
, name.fact_value name
, surname.fact_value surname
, city.fact_value city
FROM customers c
LEFT
JOIN customers_facts name
ON name.customer_id = c.customer_id
AND name.fact_name = 'name'
LEFT
JOIN customers_facts surname
ON surname.customer_id = c.customer_id
AND surname.fact_name = 'surname'
LEFT
JOIN customers_facts city
ON city.customer_id = c.customer_id
AND city.fact_name = 'city';
UPDATE my_pivot SET name = 'Leonardo' WHERE customer_id = 1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
SELECT * FROM customers_facts;
+-------------+-----------+------------+
| customer_id | fact_name | fact_value |
+-------------+-----------+------------+
| 1 | city | Milan |
| 1 | name | Leonardo |
| 2 | name | Orlando |
| 2 | surname | Bloom |
+-------------+-----------+------------+
...but this still cannot use an index.
EDIT: To answer the question asked in comments below your question, you can do...
SELECT customer_id
FROM customers_facts
WHERE
( fact_name,fact_value ) IN (('name','Orlando'),('surname','Bloom'))
GROUP
BY customer_id
HAVING COUNT(*) = 2;
...although I think MySQL can't use an index in this instance, so the longhand version might be better...
SELECT customer_id
FROM customers_facts
WHERE
( fact_name = 'name'
AND fact_value = 'Orlando'
)
OR
( fact_name = 'surname'
AND fact_value = 'Bloom'
)
GROUP
BY customer_id HAVING COUNT(*) = 2;