Conditionally select some parts of table - mysql

Question

I'm trying to conditionally select some columns of my table.
The structure of my table could look weird, but I have no influence on that:
| id | col1|1 | col2|1 | col1|2 | col2|2 | col1|3 | col2|3 |
|:--:|:------------:|:------------:|:------------:|:------------:|:------------:|:------------:|
| 1 | some | meaningless | text | don't | mind | me |
| 2 | abc | def | NULL | NULL | my | text |
| 3 | dummytext... | dummytext... | dummytext... | dummytext... | dummytext... | dummytext... |
This table is divided into 3 parts, marked with a |X at the end.
col1|1 and col2|1
col1|2 and col2|2
col1|3 and col2|3
I only want each part, if col2 of that part IS NOT NULL.
This is my approach:
SELECT t1.`col1|1`, t1.`col2|1`, t2.`col1|2`, t2.`col2|2`, t3.`col1|3`, t3.`col2|3`
FROM tab1 t1
LEFT JOIN tab1 t2 On t1.`id` = t2.`id`
LEFT JOIN tab1 t3 on t1.`id` = t3.`id`
WHERE
t1.`col2|1` IS NOT NULL
AND t2.`col2|2` IS NOT NULL # this column is NULL, so I don't want it (including table t2)
AND t3.`col2|3` IS NOT NULL
AND t1.`id` = 2
AND t2.`id` = 2
AND t3.`id` = 2
If works only if all col2 are NOT NULL, but if 1 of them IS NULL, the whole result is empty.
If you replace the both NULL-values in my example table, you would get all 6 columns, which would be right, as no part would be NULL in this case.
In my example, I want that output:
| col1|1 | col2|1 | col1|3 | col2|3 |
|:------:|:------:|:------:|:------:|
| abc | def | my | text |
Here is a fiddle.

Answer 1

I modified your code:
SELECT t1.`col1|1`, t1.`col2|1`, t2.`col1|2`, t2.`col2|2`, t3.`col1|3`, t3.`col2|3`
FROM
tab1 t
LEFT JOIN tab1 t1 On t.`id` = t1.`id` AND t1.`col2|1` IS NOT NULL
LEFT JOIN tab1 t2 On t.`id` = t2.`id` AND t2.`col2|2` IS NOT NULL
LEFT JOIN tab1 t3 on t.`id` = t3.`id` AND t3.`col2|3` IS NOT NULL
WHERE
t.`id` = 2
It realizes the described logic, but doesn't exclude the NULL columns, the result of the query:
+----+--------+--------+--------+--------+--------+--------+
| | col1|1 | col2|1 | col1|2 | col2|2 | col1|3 | col2|3 |
+----+--------+--------+--------+--------+--------+--------+
| 1 | abc | def | NULL | NULL | my | text |
+----+--------+--------+--------+--------+--------+--------+

Answer 2

In MySQL, a query can not generate the dynamic number of columns in its result. So it's not possible to conditionally select some columns of a table. The result of your query will always return six columns.
But you could try to select all 3 parts of the table. For each part, if col2 of that part is NULL then col1 of that part will be NULL also.
SELECT IF(`col2|1` IS NULL, NULL, `col1|1`) AS `col1|1`, `col2|1`,
IF(`col2|2` IS NULL, NULL, `col1|2`) AS `col1|2`, `col2|2`,
IF(`col2|3` IS NULL, NULL, `col1|3`) AS `col1|3`, `col2|3`
FROM tab1
WHERE `id` = 2