Question

Can anyone please explain me how can we use join on the basis of condition.
Lets say i am filtering data on the basis of a condition now my concern is if a particular BIT type parameters value is 1 then the data set include one more join else return same as earlier.
Here is three tables A,B,C
now i want to make a proc which has the @bool bit parameter

if @bool=0
then  
select A.* from A 
inner join B on B.id=A.id
and if @bool=1 
then 
select A.* from A
INNER JOIN B on B.id=A.id
inner join C on C.id=A.id 

Thanks In Advance.

Answer 1

What you have will work (certainly in a SPROC in MS SQL Server anyway) with minor mods.

if @bool=0 then  
   select A.* from A 
   inner join B on B.id=A.id
else if @bool=1 then -- Or just else if @boll is limited to [0,1]
   select A.* from A
   INNER JOIN B on B.id=A.id
   inner join C on C.id=A.id 

However, the caveat is that SQL parameter sniffing will cache a plan for the first path it goes down, which won’t necessarily be optimal for other paths through your code.

Also, if you do take this ‘multiple alternative query’ approach to your procs, it is generally a good idea to ensure that the column names and types returned are identitical in all cases (Your query is fine because it is A.*).

Edit
Assuming that you are using SQL Server, an alternative is to use dynamic sql:

DECLARE @sql NVARCHAR(MAX)
SET @sql = N'select A.* from A 
             inner join B on B.id=A.id'

IF @bool = 1
    SET @sql = @sql + N' inner join C on C.id=A.id'

sp_executesql @sql

If you need to add filters etc, have a look at this post: Add WHERE clauses to SQL dynamically / programmatically