I have to join tables based on conditions

table : main
condition_field
unique_id
username

table : a
unique_id
companyname

table : b
username
companyname

If main.condition_field = "X"
I need to Left Join table a on main.unique_id = a.unique_id


Else if main.condition_field = "Y"
I need to Left Join table b on main.username = b.username

So far I only have this query joining 2 table main and table a

SELECT main.*, a.* FROM main LEFT JOIN a ON main.unique_id = a.unique_id where main.unique_id='X'

Please Help