How to optimize conditional sub-selects that involve inner joins?

Question

I have got an SQL query (MS SQL 2008) that I would like to optimize speed-wise. It has got the following structure (only in reality there are 10 different when-cases in the case statement).

The important bits are the sub selects in the case statement that involve inner joins between additional tables and a reference to one of the tables in the FROM clause (table1).

I was thinking that I could optimize this using left (outer) joins in the FROM clause instead of sub select, but am not sure because the sub selects also involve inner joins as well. Would I then use two left joins in the FROM clause where now I am using an inner join in the sub selects? And how would that work with AnotherTable3 from the second when-case?

Any ideas are much appreciated.

    SELECT table1.a,
           table2.b,
           CASE
             WHEN table1.XY = 1 THEN
               (SELECT something
                FROM   AnotherTable1 at1
                       INNER JOIN AnotherTable2 at2
                               ON at1.x = at2.y
                WHERE  at1.abc = table2.abc)
             WHEN table1.XY = 2 THEN
               (SELECT something
                FROM   AnotherTable1 at1
                       INNER JOIN AnotherTable3 at3
                               ON at1.x = at3.y
                WHERE  at1.abc = table2.abc)
           END AS [problem]
    FROM   MyTable1 table1
           INNER JOIN MyTable2 table2
                   ON table1.a = table2.b 

Answer 1

In this special case where AnotherTable1 is joined in both when parts with the same condition you only need three left joins:

SELECT table1.a,
       table2.b,
       CASE
         WHEN table1.XY = 1 THEN
           ?.something -- from whereever it's coming
         WHEN table1.XY = 2 THEN
           ?.something -- from whereever it's coming
       END AS [problem]
FROM   MyTable1 table1
       INNER JOIN MyTable2 table2
               ON table1.a = table2.b 
       LEFT JOIN AnotherTable1 at1
               ON at1.abc = table2.abc
       LEFT JOIN AnotherTable2 at2
               ON at1.x = at2.y
       LEFT JOIN AnotherTable3 at3
               ON at1.x = at3.y

In more general case you would have this option

FROM   MyTable1 table1
       INNER JOIN MyTable2 table2
               ON table1.a = table2.b 
       LEFT JOIN 
            (select *
               from AnotherTable1 at1
              INNER JOIN
                    AnotherTable2 at2
                      ON at1.x = at2.y   
            ) at1
               ON at1.abc = table2.abc
       ...

Answer 2

You can try following:

SELECT 
    table1.a,
    table2.b,
    CASE 
        WHEN table1.XY = 1 THEN at2.something
        WHEN table1.XY = 2 THEN at3.something
    END
FROM   MyTable1 table1
INNER JOIN MyTable2 table2
    ON table1.a = table2.b 
LEFT JOIN AnotherTable1 at1
    ON at1.abc = table2.abc
INNER JOIN AnotherTable2 at2
    ON at1.x = at2.y
INNER JOIN AnotherTable3 at3
    ON at1.x = at3.y

But check results, also keep in mind that at2 and at3 columns must have same data type