Efficient way to join two tables with one to many relationship
Most effective way is to use an INNER JOIN, like this:

SELECT   *
FROM        Parent P
INNER JOIN  Child  C
    ON      C.ParentId = P.ParentId


And when you need aggregates on child level, you can add a GROUP BY and SUM and/or AVG or other aggregate functions:

SELECT      P.Name
            , P.Address
            , SUM(C.Amount)
FROM        Parent P
INNER JOIN  Child  C
    ON      C.ParentId = P.ParentId
GROUP BY    P.Name
            , P.Address