conditional-media-query-mixins.css

.sidebar {
  width: 33.33%;
}
@media (max-width: 600px) {
  .sidebar {
    width: 100%;
  }
}
@media (min-width: 1250px) {
  .sidebar {
    width: 20%;
  }
}

conditional-media-query-mixins.scss

// ----
// Sass (v3.3.0.rc.2)
// Compass (v1.0.0.alpha.17)
// ----
// http://css-tricks.com/conditional-media-query-mixins/
// http://paranoida.github.io/sass-mediaqueries/
// https://raw.github.com/paranoida/sass-mediaqueries/master/_media-queries.scss
@mixin bp($point) {
  @if $point == large-bp {
    @media (min-width: 1600px) { @content; }
  }
  @else if $point == medium-bp {
    @media (min-width: 1250px) { @content; }
  }
  @else if $point == small-bp {
    @media (max-width: 600px)  { @content; }
  }
}

// example of usage 
.sidebar {
  width: 33.33%;
  @include bp(small-bp) {
    width: 100%;
  }
  @include bp(medium-bp) {
    width: 20%;
  }
}