Mysql - 如何根据第一个表的 2 列的值显示第二个表中的列？

Question

I have two tables, measure:

id        currentgroup  nextgroup
1         1             2
2         2             5
3         3             24
4         99            NULL

description:

id       desc
1        First Level
2        Second Level
3        Third Level
4        Supervisor
5        Team Leader
.
.
99       CEO

and I would like to have this output

id       currentgroup   nextgroup
1        First Level    Second Level
2        Second Level   Team Leader
3        Third Level    DB department
4        CEO

These are the queries I tried. To display first two columns, this one is working:

SELECT id, description.desc
FROM measure 
INNER JOIN description ON measure.currentgroup=description.desc
WHERE measure.id IN (1,2,3,4);

QUESTION: I'm unable to add the second column. I don't know how to define description.desc for the second column (nextgrouplevel). I added second join condition, but how to show this second column 'nextgroup'?

SELECT id, description.desc
FROM measure 
INNER JOIN description ON measure.currentgroup=description.desc AND measure.nextgroup=description.desc
WHERE measure.id IN (1,2,3,4);

Answer 1

You can join twice, once for each group. Since you have some measures without a next group, the second join should be a left join .

select 
    m.id,
    d1.desc current_group,
    d2.desc next_group
from 
    measure m
    inner join description d1 on d1.id = m.current_group
    left  join description d2 on d2.id = m.next_group
where m.id in (1, 2, 3, 4)