How do I look for matching data in SQL? - sql-server

Question

I have a database table for a todo list application, and i need a way to track tasks which are dependant on other tasks, I already have a table with ID,title, description, IsComplete and a DependsOnTask column, containing the unique identifier for the task another given task is dependant on.
the problem is, when I try the below in SQL it doesn't give any results!
SELECT TOP 1000 [id]
,[title]
,[description]
,[complete]
,[DependsOnTask]
FROM [master].[dbo].[ToDoItems] where ToDoItems.id =ToDoItems.DependsOnTask;
So my question is, is there a way to find all records with a unique identifier matching DependsOnTask?
Thanks in advance :)

Answer 1

You are missing a JOIN:
SELECT tdi.*, dot.*
FROM dbo.ToDoItems tdi JOIN
dbo.ToDoItems dot
ON dot.id = tdi.DependsOnTask;
This returns all tasks where DependsOnTask is not null, along with information from that record.
Notes:
You don't need to use square braces when they are not necessary. They just clutter up queries.
Use table aliases and qualify column names, so you know where columns are coming from.
You need to use an explicit JOIN for references back to the same table.

Answer 2

If you have a hierarchical structure and task could have a parent and that parent is a child to another task, yo can use recursive CTE to find all hierarchy of determined task.
Let me show an example.
You got structure like this:
SELECT *
FROM (VALUES
(1,'Title1','Do some stuff 1', 0, NULL),
(2,'Title2','Do some stuff 2', 0, NULL),
(3,'Title3','Do some stuff 3', 1, 1),
(4,'Title4','Do some stuff 4', 1, 1),
(5,'Title5','Do some stuff 5', 0, 2),
(6,'Title6','Do some stuff 6', 1, 2),
(7,'Title7','Do some stuff 7', 0, 4),
(8,'Title8','Do some stuff 8', 0, NULL)
) as t([id],[title],[description],[complete],[DependsOnTask])
So task 1 has 2 child tasks - 3 and 4. The 4th task got 1 child - 7. You want to get all child tasks of task with id = 1:
DECLARE #taskid int = 1
;WITH cte AS (
SELECT [id]
,[title]
,[description]
,[complete]
,[DependsOnTask]
FROM [ToDoItems]
WHERE [id] = #taskid
UNION ALL
SELECT t.*
FROM [ToDoItems] t
INNER JOIN cte c
ON c.id = t.DependsOnTask
)
SELECT *
FROM cte
Output:
id title description complete DependsOnTask
1 Title1 Do some stuff 1 0 NULL
3 Title3 Do some stuff 3 1 1
4 Title4 Do some stuff 4 1 1
7 Title7 Do some stuff 7 0 4
So if you change last select to:
SELECT #taskid as main,
id,
DependsOnTask
FROM cte
You will get:
main id DependsOnTask
1 1 NULL
1 3 1
1 4 1
1 7 4
So you get all child tasks of Task1.
If you change CTE like this:
;WITH cte AS (
SELECT [id]
,[title]
,[description]
,[complete]
,[DependsOnTask]
,[id] as Parent
FROM [ToDoItems]
WHERE [DependsOnTask] IS NULL
UNION ALL
SELECT t.*,
c.Parent
FROM [ToDoItems] t
INNER JOIN cte c
ON c.id = t.DependsOnTask
)
SELECT Parent,
id,
DependsOnTask
FROM cte
You will got all you need: Parent task, Child tasks and what are they dependent on:
Parent id DependsOnTask
1 1 NULL
2 2 NULL
8 8 NULL
2 5 2
2 6 2
1 3 1
1 4 1
1 7 4