All fields present
@article{key,
    author  = {a},
    title   = {b},
    journal = {c},
    year    = {d}
}

Missing author and year fields are missing
@article{key,
    title   = {b},
    journal = {c}
}

Missing title field
@article{key,
    author  = {a},
    journal = {c},
    year    = {e}
}

Missing year field
@article{key,
    author  = {a},
    title   = {b},
    journal = {c},
}

Missing journal field
@article{key,
    author = {a},
    title  = {b},
    year   = {c}
}

All fields present
@book{key,
    publisher = {a},
    title     = {b},
    year      = {d}
}

Missing title field
@book{key,
    publisher = {a},
    year   = {b}
}

Missing year and publisher fields
@book{key,
    title = {b}
}

Missing publisher field
@book{key,
    year  = {a},
    title = {b}
}

All fields present
@booklet{key,
    title  = {a}
}

Missing title field
@booklet{key,
}

Only optional year missing
@inbook{key,
    author = {a},
    title  = {b}
}
