how to auto merge tr header

hi every one.
please help me to generate table header like this image. 
![36896002-7a66a524-1e43-11e8-8442-9c9361577892](https://user-images.githubusercontent.com/21234988/37750638-abf8840e-2dc0-11e8-8bd8-246679d8cd8f.png)
