option_1.go

type Foo struct {
    Name   string
    Number interface{} // Optional field
}

// SQL
CREATE TABLE foo (
    name VARCHAR(255),
    number INTEGER // Sometimes it is actually NULL
)

option_2.go

type Foo struct {
    Name   string
    Number int // Optional field
}

// SQL
CREATE TABLE foo (
    name VARCHAR(255),
    number INTEGER DEFAULT 0
)