Complicated SELECT - mysql

Question

Maybe I'm going in the wrong direction with this, so I'd love some pointers on how to approach this differently.
I have multiple MySql database tables with 50+ TEXT fields (categories) each (reason, problem, cause, disposition ... etc.), some (most) of these are null, some aren't, depending on the item they are describing.
I want to convert these to a 'topic', 'text' table instead. for example
item, reason , problem , cause , disposition
0001, null , broken widget, unknown , null
0002, returned, null , customer unhappy, in transit
to
item, topic , text
0001, problem , broken widget
0001, cause , unknown
0002, reason , returned
0002, cause , customer unhappy
0002, disposition, in transit
So, I'm fiddling with a select query, this is what I got so far:
SELECT item, problem AS 'text', 'problem' AS 'topic' FROM my_table
WHERE problem IS NOT NULL;
how would I go about adding the next category? Like
SELECT item, cause AS 'text', 'cause' AS 'topic' FROM my_table
WHERE cause IS NOT NULL;
1: how would I add that to the previous select statement so that it would cover both 'cause' and 'problem'?
2: since there are so many categories, is there a way to avoid the manual labor here and iterate through field names somehow?

Answer 1

You need to be very careful about using single quotes in queries. A simple rule: only use single quotes for string and date constants. Otherwise, the escape character is the backtick -- and if you name columns and tables correctly, you don't need that.
So this query:
SELECT item, cause AS 'text', 'cause' AS 'topic'
FROM my_table
WHERE cause IS NOT NULL;
Should be:
SELECT item, cause AS text, 'cause' AS topic
FROM my_table
WHERE cause IS NOT NULL;
Then, you can combine these using union all:
SELECT item, problem AS text, 'problem' AS topic FROM my_table WHERE problem IS NOT NULL UNION ALL
SELECT item, cause AS text, 'cause' AS topic FROM my_table WHERE cause IS NOT NULL
And, you can keep adding subqueries for all the columns you want to include this way.
EDIT:
If you want to do this with information_schema.columns, you can use replace:
select replace('SELECT item, <colname> AS text, ''<colname>'' AS topic FROM my_table WHERE problem IS NOT NULL UNION ALL',
'<colname>', column_name)
from information_schema.columns c
where table_name = XXX and
column_name <> 'item';

Answer 2

You can use UNION ALL:
SELECT item, problem AS text, 'problem' AS topic
FROM my_table
WHERE problem IS NOT NULL
UNION ALL
SELECT item, cause AS text, 'cause' AS topic
FROM my_table
WHERE cause IS NOT NULL;
To save some time building the query you could use INFORMATION_SCHEMA to list the columns:
SELECT `COLUMN_NAME`
FROM `INFORMATION_SCHEMA`.`COLUMNS`
WHERE `TABLE_SCHEMA`='yourdatabasename'
AND `TABLE_NAME`='my_table';
Either concatenate the query with the COLUMN_NAME or just paste the list of column names into Excel and build the query there and copy/down.