How should I install the overflow hole of the washbasin downcomer?

How to install the overflow hole of the washbasin downcomer is as shown in the picture, connect the main sewer pipe, and then connect the basin overflow to the main sewer pipe with the pipeline.

The purpose of the overflow port is to prevent the water level of the basin from being too high, and the extra water can flow away through the piping through the basin overflow hole and the sewer main overflow port.

With overflow holes can be used in the basin without overflow holes, provided that the overflow holes can not be exposed, and the top and bottom of the water seal should be tight, otherwise the water will run out of the overflow holes.

Without overflow holes can also be used in the basin with overflow holes, but then the basin overflow holes lose their role.

Extended information

It's not "overflow", it's called "overflow hole". Your basin is a ceramic basin, there is a round hole ----- water overflow from here to drain out, through the "overflow hole" to reach the sewer.