4- @for && @if-@else

1- @for

@for $i from 1 through 12 {
    .col-#{$i}{
        width:$i/12*100%
    }
}

2- @if-@else

@mixin  v-h($val:true) {
    @if $val {
        display: block;
    }@else{
        display: none;
    }  
}
.v{
    @include v-h;
}
.h{
    @include v-h(false)
}

3- @extend继承样式

.three{
    @extend .two;  
    background-color: red;
}