How do I mysql select with aliases from another table? - mysql

Question

I'm working with a CMS system where I cannot control database column names. And I've got two related tables:
Table: content
+------------+----------+----------+----------+----------+
| content_id | column_1 | column_2 | column_3 | column_4 |
+------------+----------+----------+----------+----------+
| 1 | stuff | junk | text | info |
| 2 | trash | blah | what | bio |
+------------+----------+----------+----------+----------+
Table: column_names
+------------+-------------+
| column_id | column_name |
+------------+-------------+
| 1 | good_text |
| 2 | bad_text |
| 3 | blue_text |
| 4 | red_text |
+------------+-------------+
What I'd like to do here is select from the first table, but select the columns AS the column_name from the second table. So my result would look like:
+------------+-----------+----------+-----------+----------+
| content_id | good_text | bad_text | blue_text | red_text |
+------------+-----------+----------+-----------+----------+
| 1 | stuff | junk | text | info |
| 2 | trash | blah | what | bio |
+------------+-----------+----------+-----------+----------+

Answer 1

There is no way to do this, but as long as your columns won't change too often, a VIEW on that table with hard-coded aliases could be sufficient.

Answer 2

To my knowledge you can't use subselects as the target of an AS in SQL. If I were you, I'd load the data from column_names into an array (I'd also cache it in an in-memory DB since it isn't going to change all that often), then use that array to build my SQL.
A very rough example (in PHP):
$column_relations = array();
$sql = "SELECT * FROM column_names;";
$res = mysql_query($sql);
while ($row = mysql_fetch_assoc($res))
{
$column_relations[$row['column_name']] = 'column_' . $row['column_id'];
}
$sql = sprintf("SELECT content_id, '%s' AS good_text, '%s' AS bad_text, '%s' AS blue_text, '%s' AS red_text FROM content;", $column_relations['good_text'], $column_relations['bad_text'], $column_relations['blue_text'], $column_relations['red_text']);

Answer 3

SELECT
cn. column_id id,
MAX(IF(content_id=1,column_1,null)) as good_text,
MAX(IF(content_id=2,column_2,null)) as bad_text,
MAX(IF(content_id=3,column_3,null)) as blue_text,
MAX(IF(content_id=4,column_4,null)) as red_text
FROM content ct
INNER JOIN column_names cn ON (cn. column_id = ct.content_id)
GROUP BY cn. column_id
Sorry, it should be LEFT JOIN, not INNER.
You cannot have a dynamic number of columns, so this solution works only if you know in advance how many
groups you might have.