How can I write this SQL query correctly? - mysql

Question

I'm trying to figure out how to write a query to get the right results.
I'll keep it simple. First, the foundation:
I have two tables: DEALS and TASKS
A Deal can have 1 or more Task inside, so TASKS has a deal_id field.
Also, every Task has a time_start field (unix timestamp) and a completed field (1 or 0).
Ok. Now, what I need? In my view I need to show all deals with the "Next Task" column rendered.
So for every Deal, if I have a Task (one or more) it must show only the closest. If no Task, I'll render an alert.
Deal Title | Value | Next step
deal 1 | 1.000 | tomorrow at 11:00
deal 2 | 1.000 | NO TASK IN THIS DEAL
deal 3 | 1.500 | 12/03/2017 at 9:00
In this example, deal 1 has 3 tasks inside, but the nearest start tomorrow. I don't want "deal 1" repeated 3 times. <-- GROUP BY deals.id??
To get this right, currently, I run the deals query without JOIN and I'm using a custom PDO class to run a new query for tasks for every row.
But this is BAD! I have a new query for every row of the DEALS table.
I'm pretty sure that there is a way to write one single query to get this result.
PS: Don't care about the rendering of the text, I used "tomorrow" only to write the example, next_spet is the unix timestamp from the db ... I can easily use moment.js to format it correctly.
EDIT:
I'll provide the data inside the 2 tables, just to complete the example.
DEALS
ID | TITLE | VALUE
1 | Deal 1 | 1000
2 | Deal 2 | 1000
3 | Deal 3 | 1000
TASKS
ID | DEAL_ID | TITLE | TIME_START | COMPLETED
1 | 1 | Send Proposal | 1483678800 | 0
2 | 1 | Follow up | 1483441200 | 0
3 | 1 | Ask for referrals | 1484441200 | 0
4 | 2 | Send email | 1483678900 | 0
5 | 3 | Sort out meeting | 1483678900 | 0
NOTE: timestamps don't match with the results that I have written in the first table. They were just an example, I take the timestamp of the time_start field and format it in a human readable mode, but this isn't my question.

Answer 1

If you want the time of the next task, you can use a correlated subquery:
select d.*,
(select t.time_start
from tasks t
where t.deal_id = d.deal_id and t.time_start > d.time_col
order by t.time_start
limit 1
) as task_time_start
from deals d;
EDIT:
If you want the future, then just change the time comparison:
select d.*,
(select t.time_start
from tasks t
where t.deal_id = d.deal_id and t.completed = 0 and
t.time_start > now()
order by t.time_start
limit 1
) as task_time_start
from deals d;

Answer 2

Something like this should do it.
select
d.title as 'Deal Title',
d.value as 'Value',
IsNull(CONVERT(VARCHAR(50), min(t.time_start)), 'NO TASK IN THIS DEAL') as 'Next step'
from Deals d
left join Tasks t
on t.deal_id = d.deal_id
and t.is_completed = 0
and t.time_start > GetDate()
group by d.deal_id, d.title, d.value
Here's the SQLite equivalent:
select
d.title as 'Deal Title',
d.value as 'Value',
IfNull(min(t.time_start), 'NO TASK IN THIS DEAL') as 'Next step'
from Deals d
left join Tasks t
on t.deal_id = d.deal_id
and t.is_completed = 0
and t.time_start > now() --remove this line to include past Tasks
group by d.deal_id, d.title, d.value