JOIN TWO TABLES [duplicate] - mysql

Question

This question already has answers here:
How can I return pivot table output in MySQL?
(10 answers)
Closed 8 years ago.
I have two tables: 'Project' and 'Project Monthly'.
| PROJECT_ID | TITLE | | ID | PROJECT_ID | MONTH | EXPENSE |
----------------------- --------------------------------------
| 1 | title1 | | 1 | 1 | 1 | 100 |
| 2 | title2 | | 2 | 1 | 2 | 2000 |
| 3 | title3 | | 3 | 1 | 3 | 900 |
| 4 | 1 | 4 | 900 |
| 5 | 2 | 1 | 200 |
| 6 | 2 | 2 | 200 |
| 7 | 3 | 1 | 500 |
I would like to have a table like this:
| PROJECT_ID | TITLE | MONTH_1 | MONTH_2 | MONTH_3 | MONTH_4 |
---------------------------------------------------------------
| 1 | title1 | 100 | 2000 | 900 | 900 |
| 2 | title2 | 200 | 200 | NULL| NULL|
| 3 | title3 | 500 | NULL | NULL| NULL|
Can I realize it only with JOIN and without subqueries?
Thanks!

Answer 1

Turning rows into columns is called pivoting. One approach is to group by each project. The group will become a single row. Inside the group, you define one column per month. The column sums up the expenses for one particular month:
select p.PROJECT_ID
, p.TITLE
, sum(case when m.month = 1 then m.expense end) as MONTH_1
, sum(case when m.month = 2 then m.expense end) as MONTH_2
...
from project p
join monthly m
on p.PROJECT_ID = m.PROJECT_ID
group by
p.PROJECT_ID
, p.TITLE

Answer 2

try like this
select p.PROJECT_ID,p.TITLE,
case when month=1 then EXPENSE end as Month_1,
case when month=2 then EXPENSE end as Month_2,
case when month=3 then EXPENSE end as Month_3,
case when month=4 then EXPENSE end as Month_4
from Project p inner join ProjectMonthly' pm on p.PROJECT_ID=pm.PROJECT_ID

Answer 3

You can do so by using case with max to get the desired result set,below query will only give the results for 4 months if you have more than 4 months then you have to write cases for all months with a max to pick greater value among the values for a month,i have used inner join so the projects that exists in ProjectMonthly only these projects will be returned if any project has no monthly data and you still want it to return the results the change inner join to left join
SELECT p.PROJECT_ID,p.TITLE,
MAX(CASE WHEN MONTH=1 THEN EXPENSE END) AS Month_1,
MAX(CASE WHEN MONTH=2 THEN EXPENSE END) AS Month_2,
MAX(CASE WHEN MONTH=3 THEN EXPENSE END) AS Month_3,
MAX(CASE WHEN MONTH=4 THEN EXPENSE END) AS Month_4
FROM Project p
INNER JOIN ProjectMonthly pm
ON p.PROJECT_ID=pm.PROJECT_ID
GROUP BY p.PROJECT_ID
Fiddle Demo