SQL & PL/SQL :: How To Convert A Single Ragged Table

how to convert a single 'ragged' table like:

MAIN_KEY SUB_KEY DATA
ABC 1 A
ABC 2 B
ABC 3 C
DEF 1 X

where:

MAIN_KEY and DATA are indeterminate, and
SUB_KEY = 1 .. n [always starts with 1 for a given MAIN_KEYD]

into:

MAIN_KEY SUB_KEY DATA
ABC 1 A
ABC 2 B
ABC 3 C
ABC 4 {null}
ABC 5 {null}
DEF 1 X
DEF 2 {null}
DEF 3 {null}
DEF 4 {null}
DEF 5 {null}

in the case where n = 5?