Multiple GROUP_CONCATs in one query - mysql

Question

I have two tables:
CREATE TABLE instructions (
`id_instruction` INT(11),
`id_step` INT(11)
);
CREATE TABLE steps (
`id_instruction` INT(11),
`id_step` INT(11),
`val` VARCHAR(255)
);
One table contains instructions, another table contains steps. Each instruction may have many steps. Now, the data is:
INSERT INTO instructions (`id_instruction`, `id_step`) VALUES (1, 0), (1, 1), (1, 2);
INSERT INTO steps (`id_instruction`, `id_step`, `val` ) VALUES (1, 0, 'One'), (1, 0, 'Two'), (1, 0, 'Three'); /* step 0 */
INSERT INTO steps (`id_instruction`, `id_step`, `val` ) VALUES (1, 1, 'Five'), (1, 1, 'Six'), (1, 1, 'Seven'); /* step 1 */
INSERT INTO steps (`id_instruction`, `id_step`, `val` ) VALUES (1, 2, 'Eight'), (1, 2, 'Nine'), (1, 2, 'Ten'); /* step 2 */
For each instruction I want to have two concatenations - one which concatenates values from val column for the zero step, and another one which concatenates values from the same column for the largest step of the instruction. I know how to get the largest step and how to make a single group concatenation, but trying to do two concatenations, I get duplicates. Now, my query looks like this:
SELECT maxstep, i.id_instruction, i.id_step, GROUP_CONCAT(s.val) AS val_0
FROM instructions i
INNER JOIN (
SELECT MAX(id_step) AS maxstep, id_instruction FROM instructions i
GROUP BY i.id_instruction
) i2 ON i2.id_instruction = i.id_instruction
LEFT JOIN steps s ON s.id_instruction = i.id_instruction AND s.id_step = i.id_step
GROUP BY i.id_instruction, i.id_step
It just concatenates values per a pair instruction-step. But I want to have one more concatenation which would also concatenate values for the maxstep. The desired result should look like this:
| maxstep | id_instruction | val_0 | val_1 |
| 2 | 1 | One,Two, Three | Eight, Nine, Ten |
PS. I do join instead of just MAX and grouping, because I want to use its value in additional joining for further concatenation.

Answer 1

What you're trying to do is called pivoting. In MySQL there's no built-in function for this, but you can do it like this:
SELECT maxstep, id_instruction,
MAX(CASE id_step WHEN 0 THEN val END) AS val_0,
MAX(CASE id_step WHEN 1 THEN val END) AS val_1,
MAX(CASE id_step WHEN 2 THEN val END) AS val_2
FROM (
SELECT maxstep, i.id_instruction, i.id_step, GROUP_CONCAT(s.val) AS val
FROM instructions i
INNER JOIN (
SELECT MAX(id_step) AS maxstep, id_instruction FROM instructions i
GROUP BY i.id_instruction
) i2 ON i2.id_instruction = i.id_instruction
LEFT JOIN steps s ON s.id_instruction = i.id_instruction AND s.id_step = i.id_step
GROUP BY i.id_instruction, i.id_step
) sq
GROUP BY maxstep, id_instruction
Result:
maxstep id_instruction val_0 val_1 val_2
-----------------------------------------------------------------
2 1 One,Two,Three Five,Six,Seven Ten,Eight,Nine

Answer 2

By changing the query a little so that the inner join only gets the highest step and by setting the outer query to only take id_step=0 you can get what you want.
SELECT maxstep, i.id_instruction,GROUP_CONCAT(s.val) AS val_0, val_1
FROM instructions i
INNER JOIN (
SELECT MAX(ins.id_step) AS maxstep, ins.id_instruction, GROUP_CONCAT(st.val) as val_1 FROM instructions ins
LEFT JOIN steps st ON st.id_instruction = ins.id_instruction AND st.id_step = ins.id_step
where (ins.id_instruction, ins.id_step) in (select id_instruction, max(id_step) from instructions group by id_instruction)
GROUP BY ins.id_instruction, ins.id_step
order by maxstep, ins.id_instruction, st.val
)
i2 ON i2.id_instruction = i.id_instruction
LEFT JOIN steps s ON s.id_instruction = i.id_instruction AND s.id_step = i.id_step
where i.id_step=0
GROUP BY i.id_instruction, i.id_step;
Result from the query with extended data now looks like
| maxstep | id_instruction | val_0 | val_1 |
| 2 | 1 | One,Two,Three | Eight,Nine,Ten |
| 3 | 2 | One,Two,Three | 21,22,23 |