A root canal is done on a tooth that has damage affecting the nerve. The nerve and surrounding tissue are removed, causing the tooth to "die" and weaken. A crown is placed over the tooth to prevent it from cracking or being damaged in any way.
Usually that is the case when installing crowns. The dentist will remove the infected area, remove the nerves, fill the tooth and then finally add the crown.
If root canal is necessary, yes. But if the nerve is healthy, no. If root canal is needed later, it can be done through the crown, but could damage crown.
In general, a root canal must be performed prior to a crown being installed. A root canal cleans out the area completely, minimizing the risk of infection.
