How to display lines as dynamic columns with other values? - mysql

Question

I want to display orders item that have the collect_id = 2
And I want to display all the fields related to each order_item as columns with values.
These are the tables and the result :
+-------------------------------+
| order_item |
+-------------------------------+
| oi_id oi_price oi_collect_id |
| 1 100 2 |
| 2 30 2 |
| 3 55 3 |
| 4 70 4 |
| 5 220 2 |
| 6 300 4 |
+-------------------------------+
+-----------------------------------+
| field_value |
+-----------------------------------+
| v_value v_fk_field_id oi_fk_id |
| Peter 1 1 |
| Lagaf 2 1 |
| Football 3 1 |
| Male 4 1 |
| 12345678 5 1 |
| Frank 1 2 |
| Loran 2 2 |
| Tennis 3 2 |
| Male 4 2 |
| 11223658 5 2 |
| Nathali 1 5 |
| Waton 2 5 |
| Reading 3 5 |
+-----------------------------------+
oi_fk_id : foreign key ref(order_item.oi_id)
v_fk_field_id : foreign key ref(field.f_id)
+--------------------+
| field |
+--------------------+
| f_id f_label |
| 1 surname |
| 2 name |
| 3 hobbies |
| 4 sex |
| 5 phone |
+--------------------+
+-----------------------------------------------------------------------------+
| Result |
+-----------------------------------------------------------------------------+
| oi_id oi_price oi_collect_id surname name hobbies sex phone |
| 1 100 2 Peter Lagaf Football Male 12345678 |
| 2 30 2 Frank Loran Tennis Male 11223658 |
| 5 220 2 Nathali Waton Reading null null |
+-----------------------------------------------------------------------------+
Important : The table field does not contain only these 5 fields (name, surname, hobbies, sex, phone), but it can contain many others, that the developper may not know, same thing for the correspondant value on the table 'field_value'.
PS : I didn't make field labels as columns in a table because they are dynamic and not limited, and in the front end application, the user can add new fields as he want.

Answer 1

You can take advantage of dynamic pivoting to get the results:
SELECT GROUP_CONCAT(t.line)
FROM (
SELECT CONCAT('MAX(IF(t.l=''', f.f_label, ''',t.v,NULL)) AS ', f.f_label) AS line
FROM field f
) AS t
INTO #dynamic;
SELECT CONCAT('SELECT t.oi_id, t.oi_price, t.oi_collect_id,',
#dynamic,
' FROM ( SELECT oi.*, f.f_label AS l, fv.v_value AS v FROM order_item oi JOIN field_value fv ON fv.oi_fk_id = oi.oi_id JOIN field f ON f.f_id = fv.v_fk_field_id WHERE oi.oi_collect_id = 2 ) AS t GROUP BY t.oi_id;')
INTO #sql;
PREPARE stmt FROM #sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
However it is limited by GROUP_CONCAT function:
The result is truncated to the maximum length that is given by the group_concat_max_len system variable, which has a default value of 1024. The value can be set higher, although the effective maximum length of the return value is constrained by the value of max_allowed_packet.
EDIT - Why MAX function is required?
To solve the given problem, we are creating a dynamic query, based on the content of the field table.
For the given exemplary data, the query without use of MAX function would be:
SELECT t.oi_id,
t.oi_price,
t.oi_collect_id,
IF(t.l='surname', t.v, NULL) AS surname,
IF(t.l='name', t.v, NULL) AS name,
IF(t.l='hobbies', t.v, NULL) AS hobbies,
IF(t.l='sex', t.v, NULL) AS sex,
IF(t.l='phone', t.v, NULL) AS phone
FROM (
SELECT oi.*,
f.f_label AS l,
fv.v_value as V
FROM order_item oi
JOIN field_value fv
ON fv.oi_fk_id = oi.oi_id
JOIN field f
ON f.f_id = fv.v_fk_field_id
WHERE oi.oi_collect_id = 2
) AS t;
Which wouldd result in:
+-------+----------+---------------+---------+-------+----------+------+----------+
| oi_id | oi_price | oi_collect_id | surname | name | hobbies | sex | phone |
+-------+----------+---------------+---------+-------+----------+------+----------+
| 1 | 100 | 2 | Peter | NULL | NULL | NULL | NULL |
| 2 | 30 | 2 | Frank | NULL | NULL | NULL | NULL |
| 5 | 220 | 2 | Nathali | NULL | NULL | NULL | NULL |
| 1 | 100 | 2 | NULL | Lagaf | NULL | NULL | NULL |
| 2 | 30 | 2 | NULL | Loran | NULL | NULL | NULL |
| 5 | 220 | 2 | NULL | Waton | NULL | NULL | NULL |
| 1 | 100 | 2 | NULL | NULL | Football | NULL | NULL |
| 2 | 30 | 2 | NULL | NULL | Tennis | NULL | NULL |
| 5 | 220 | 2 | NULL | NULL | Reading | NULL | NULL |
| 1 | 100 | 2 | NULL | NULL | NULL | Male | NULL |
| 2 | 30 | 2 | NULL | NULL | NULL | Male | NULL |
| 1 | 100 | 2 | NULL | NULL | NULL | NULL | 12345678 |
| 2 | 30 | 2 | NULL | NULL | NULL | NULL | 11223658 |
+-------+----------+---------------+---------+-------+----------+------+----------+
This is an intermediate result, where each row consists of value for one field and NULL for the others. The MAX function together with a GROUP BY clause is used to combine multiple rows concerning one order item in such a way, that it chooses non null values. It could be replaced by MIN function, which will also favor existing values over null.