父元素min-height:100px 子元素 height:100%无效

<div id="div1">
    <span id="sp1">aaa</span>
</div>

#div1{
     min-height:100px;
     background-color: yellow;
     position: relative;
}
#sp1{
     width: 20%;
     height: 100%;
     background-color: blue;
     position: absolute;
}