联接查询以选择表

Question

I just started learning about Join in SQL. I have gone through various online and offline tutorial to understand it. Then I started doing some questions to find out how much I understand. Below are the tables

1) ID NUMBER
2) NAME VARCHAR2


1)ID NUMBER
2)NAME VARCHAR2


1)ID NUMBER
2)CUSTOMER_ID NUMBER (NULL for internal projects)
3)NAME VARCHAR2 4)TYPE NUMBER (0 - Scrum, 1 - Fixed price, 2 - Proof of Concept)
5)START_DATE DATE (NULL when not started yet)
6)END_DATE DATE (NULL when not finished yet)


1) ID NUMBER
2)NAME VARCHAR2
3)PROJECT_ID NUMBER
4)TYPE NUMBER (0 - Deployment, 1 - Support, 2 - Ticket, 3 – Task)
5)START_DATE DATE (NULL when not started yet)
6)END_DATE DATE (NULL when not finished yet)


1) DEV_ID NUMBER
2)TASK_ID NUMBER


Now i want to write query for these question:

1)Which developer spent time on a project that was actually already finished and for which customer was it?
2)How many support tasks did each developer work on last year for Proof of Concept projects?
3)Which developer did not participate on a Scrum project yet?

Joins need some common attribute. Based on that we need to write queries. I am not able to understand how these tables are related and how to write queries for these?

Answer 1

Something like:

1)

SELECT * FROM Developer D WHERE D.ID IN(
   SELECT DEV_ID FROM Developer_TASK DT 
   JOIN Tasks T ON DT.TASK_ID = T.ID 
   JOIN Project P ON P.ID = T.PROJECT_ID
   WHERE P.END_DATE > T.END_DATE
)

"Check with current datetime" Already ended projects

SELECT * FROM Developer D WHERE D.ID IN(
   SELECT DEV_ID FROM Developer_TASK DT 
   JOIN Tasks T ON DT.TASK_ID = T.ID 
   JOIN Project P ON P.ID = T.PROJECT_ID
   WHERE P.END_DATE < GETDATE()
)

2)

SELECT DEV_ID, Name, COUNT(*) AS Entries FROM Developer D 
    JOIN Developer_Task DT ON D.ID = DT.DEV_ID 
    JOIN Tasks T ON TD.Task_ID = T.ID 
    GROUP BY DEV_ID, YEAR(T.START_DATE), Name

3)

SELECT * FROM Developer 
          WHERE DEV_ID NOT IN(SELECT DEV_ID FROM Developer_TASK DT 
                JOIN Tasks T ON DT.TASK_ID = T.ID 
                JOIN Project P ON P.ID = T.PROJECT_ID)

Answer 2

Which developer spent time on a project that was actually already finished and for which customer was it?

select p.id project_id, p.name project_name, d.name dev_name, 
       case p.customer_id when 0 then 'Internal' else c.name end cust_name
  from project p
    left join customer c on c.id = p.customer_id
    join task t on t.project_id = p.id
    join developer_task dt on dt.task_id = t.id
    join developer d on d.id = dt.dev_id
  where p.end_date is not null

These are basic joins. p.end_date is not null is responsible for filtering finished projects. I do not know if customer table contains row for internal project so I used case construction and left join to not lose rows for such projects.

How many support tasks did each developer work on last year for Proof of Concept projects?

select id, name, nvl(cnt, 0) cnt
  from developer d
  left join (
    select dev_id, count(1) cnt
      from project p
        join task t on t.project_id = p.id
        join developer_task dt on dt.task_id = t.id
      where p.type = 2 
        and t.start_date <= trunc(sysdate, 'year') - 1 
        and (add_months(trunc(sysdate, 'year'), -12) <= t.end_date or t.end_date is null)
      group by dev_id) t on t.dev_id = d.id

Here I grouped at first number of tasks for proof of concept projects for each developer. Date filtering is somewhat tricky, please study it carefully. Last step was to show data for all developers so I used left join to show all, even if they did not participate in such projects. They have count = 0.

Which developer did not participate on a Scrum project yet?

select * 
  from developer 
  where id not in (
    select dev_id 
      from project p 
      join task t on t.project_id = p.id 
      join developer_task dt on dt.task_id = t.id
      where p.type = 0)

Find dev_id for all Scrum projects. Then use not in . You can also use not exists , minus , analytical count . Oracle often allows to do task in several ways.

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Here is the dbfiddle demo with all working queries and sample data I imagined to have material to check if syntax is correct and if results looks OK.

Answer 3

If you want to know those questions you first will need to add a Developer foreign key in Project table .
Then, you will be able to know Developer actions in Project table.
Greetings