Determine table to join based on the condition - sql-server

Question

I am not sure if I've missed to search properly, but I couldn't get the exact question as mine
This is something similar but not exact
https://stackoverflow.com/questions/11533636/determining-which-table-to-join-to
Actually I want to decide with which table to join based on the parameter passed to stored procedure, case when didn't work
Kind of
select * from Table1
left join (case when #Parameter<>NULL Then Table2 else Table3 end) final
on Table1.Col1 = final.Col1
Table2 and Table3 has same structure

Answer 1

I can think of a few of options:
1: IF ... ELSE
IF #Parameter IS NULL
SELECT *
FROM T1
INNER JOIN T2 ON T1.ID = T2.ID
ELSE
SELECT *
FROM T1
INNER JOIN T3 ON T1.ID = T3.ID
2: Dynamic T-SQL
DECLARE #SQL NVARCHAR(MAX);
SELECT #SQL = N'SELECT *
FROM T1
INNER JOIN ' + CASE WHEN #Parameter IS NULL THEN N'T2 t' ELSE N'T3 t' END
+ N' ON T1.ID = t.ID';
EXEC sp_executesql #SQL;
3: UNION ALL and subquery.
SELECT *
FROM T1
INNER JOIN
(
SELECT *
FROM T2
WHERE #Parameter IS NULL
UNION ALL
SELECT *
FROM T3
WHERE #Parameter IS NOT NULL
) t ON T1.ID = t.ID
For this last one you'd have to check the plan the optimiser creates to see if it's OK performance wise.
It seems like you're looking for code reuse, so maybe option 2 is your best one. T-SQL doesn't really lend itself to this sort of polymorphism but you can use workarounds in some cases.
Taking a step back, one question to ask is, if the tables have the same structure, maybe you should just use one table? Then you could use #Parameter to just filter the rows you require instead of trying to create dynamic queries.
It's also worth noting that in situations like this you could handle the code-generation at the application layer with ORMs and the like.

Answer 2

Try this:
select *
from (
select data.*,
case when selection=1 then t1.ExtraField
when selection=2 then t2.ExtraField
when selection=3 then t3.ExtraField
else NULL
end as ExtraField
from data
left join t1 on t1.key = data.key and selection=1
left join t2 on t2.key = data.key and selection=2
left join t3 on t3.key = data.key and selection=3
) T
where ExtraField is not NULL

Answer 3

Done with this
SELECT PA.`module_id` as usertype, PA.module_pk_id, (
CASE PA.`module_id`
WHEN '0'
THEN pea.`name`
ELSE CONCAT(u.`first_name`, " ",u.`last_name`)
END
) as name
FROM `placements_approvers` PA
LEFT JOIN
placements_external_approvers pea
ON
PA.module_pk_id = pea.placement_external_approver_id AND PA.`module_id` = 0
LEFT JOIN
users u
ON
PA.module_pk_id = u.user_id AND PA.`module_id` != 0
WHERE PA.placement_id = '494' AND PA.approver_type = 1