Question

everyone i would like to perform a query on MySQL with dynamic rows.

Imagine this:

table phones_categories

+----+------------+
| id | name       |
+----+------------+
|  1 | Home Phone |
|  2 | Cell Phone | 
|  3 | Fax        | 
+----+------------+

table phones

+----+-----------+-------------------+--------------+
| id | entity_id | phone_category_id | phone_number |
+----+-----------+-------------------+--------------+
|  1 |         1 |                 1 | X19 XXX 2XX  |
|  2 |         1 |                 3 | X19 XXX 2XX  |
|  3 |         2 |                 1 | X18 XXX 4XX  |
|  4 |         2 |                 3 | X18 XXX 4XX  |
+----+-----------+-------------------+--------------+

i would like to have the following output:

+-----------+--------------+--------------+-------------+
| entity_id | Home Phone   | Cell Phone   | Fax         |
+-----------+--------------+--------------+-------------+
|         1 |  X19 XXX 2XX |              | X19 XXX 2XX |             
|         2 |  X18 XXX 4XX |              | X18 XXX 4XX |
+-----------+--------------+--------------+-------------+

Ok i need some “dynamic” becacuse on the future the table phone_categories can grow.

Answer 1

This is called a “pivot table” or “crosstab query”. MySQL alone cannot do this dynamically. You always need to know the column names ahead of time, so if you are using a programming/scripting language for your output you can use it to dynamically build up the SQL statement with a for loop after you’ve determined the categories.

But the query will look like:

SELECT
  phones.entity_id,
  CASE WHEN phones.phone_category_id = 1 THEN phones.phone_number ELSE NULL END AS `Home Phone`,
  CASE WHEN phones.phone_category_id = 2 THEN phones.phone_number ELSE NULL END AS `Cell Phone`,
  CASE WHEN phones.phone_category_id = 3 THEN phones.phone_number ELSE NULL END AS `Fax`
FROM phones

You haven’t identified any programming language, so here’s some pseudocode to generate the query:

categories = "SELECT id, name FROM phone_categories;"
foreach categories
  sql_columns = sql_columns + " CASE WHEN phones.phone_category_id = " + categories.id + " THEN phones.phone_number ELSE NULL END AS  `categories.name`