Question

Could you please let me know how to left join two different tables based on the condition? I appreciate any help.

SELECT A.id,
       A.flag,
       B.fname,
       B.lname
FROM   Table1 A
       CASE 
            WHEN ISNULL(A.flag, 0) = 0 THEN LEFT
       JOIN Table2 B
            ON  B.id = A.id
                ELSE
       LEFT JOIN Table3 B
            ON  B.id = A.id

Answer 1

SELECT A.id,
       A.flag,
       CASE 
            WHEN ISNULL(A.flag, 0) = 0 THEN B1.fname
            ELSE B2.fname
       END as fname,
       CASE 
            WHEN ISNULL(A.flag, 0) = 0 THEN B1.lname
            ELSE B2.lname
       END as lname
FROM   Table1 A
       LEFT JOIN Table2 B1
            ON  B1.id = A.id
       LEFT JOIN Table3 B2
            ON  B2.id = A.id

Try this:

 CASE 
        WHEN ISNULL(A.flag, 0) = 0 THEN B1.fname + ' ' + B1.lname
            ELSE B2.fname + ' ' + B2.lname
        END as name