select a.field1, a.field2, b.fieldfromb ; from table1 a ; inner join table2 b on b.somefield=a.somefield; where a.field1="X"or this:
select a.field1, a.field2, b.fieldfromb ; from table1 a ; inner join table2 b on a.field1="X" AND b.somefield=a.somefield