SQL Server left join if condition met - sql-server

Question

I have a problem with my query. I have to do left outer join only if condition is true. If condition is false do another left outer join.
I tried with this but not successfully:
select
*
from
works with(nolock)
if work.type = 1
begin
left outer join
users with(nolock) on users.id = work.owner
else
left outer join
groups with(nolock) on groups.id = work.owner
end
How can I solve this problem?

Answer 1

you should try left join both of them, but inside of the select, take what you want depending on the use case.
SELECT
*,
CASE work.type WHEN '1' THEN 'a.owner' ELSE 'b.owner' END AS owner
FROM
blahblah
left join users on blahblah.user_id = users.id as a,
left join groups as blahblah.groups_id = groups.id as b

Answer 2

You can try below query.
First method:
select
works.*, isnull(users.id, groups.id)
from
works with(nolock)
left outer join
users with(nolock) on users.id = works.owner and work.type = 1
left outer join
groups with(nolock) on groups.id = works.owner
Second method:
if exists (select 1 from works with (nolock) where works.type = 1)
select *
from works with(nolock)
left outer join users with(nolock) on users.id = works.owner
else
select *
from works with(nolock)
left outer join groups with(nolock) on groups.id = works.owner
third method: use dynamic SQL to build query at runtime.

Answer 3

Considering that you want to display all columns (*), then you can conditionally join against both tables by checking the work.type as a join condition:
select
*
from
works
left join users on
users.id = work.owner and
work.type = 1
left join groups on
groups.id = work.owner and
(work.type <> 1 OR work.type IS NULL)
A particular row from the works table can only have a particular value for type, so it will join against users or against groups, but never both. The problem with this solution becomes the displayed columns, since we are joining against both tables, now you have to unify groups and users columns.
You can do this with a bunch of ISNULLs:
select
works.*,
Column1 = ISNULL(users.Column1, groups.Column1),
Column2 = ISNULL(users.Column2, groups.Column2)
from
works
left join users on
users.id = work.owner and
work.type = 1
left join groups on
groups.id = work.owner and
(work.type <> 1 OR work.type IS NULL)
If you need to use the same select repeatedly, you can create a table-valued function to wrap this up so you don't have to code it every time. I'll use the table example to show another alternative, using UNION ALL.
CREATE FUNCTION dbo.GetWorkData (#owner INT) -- assuming its a INT
RETURNS TABLE
AS
RETURN
SELECT
-- Your wanted columns here
FROM
works AS W
INNER JOIN users AS U ON W.owner = U.owner
WHERE
W.owner = #owner AND
W.type = 1
UNION ALL
SELECT
-- Your wanted columns here (must be same data type and order of previous SELECT)
FROM
works AS W
INNER JOIN groups AS U ON W.owner = U.owner
WHERE
W.owner = #owner AND
(W.type <> 1 OR W.type IS NULL)
You can use the function with APPLY:
SELECT
D.*
FROM
works AS W
CROSS APPLY dbo.GetWorkData(W.owner) AS D -- User "OUTER APPLY" if you want works that have no users or groups

Answer 4

You can look into Dynamic SQL. The main idea is that the SQL statement is constructed and compiled at runtime.
You can start here:
Link 1 - MSSQL tips.
or here: Link 2 - official microsoft documentation.