関係がある【かんけいがある】

関係がある means "is related to", "has to do with": かんけい関係 is "connection", and ある says it exists. What it is connected to takes と or に.

と puts the two things side by side, so the link runs both ways. に points from one thing to the other, and suits "is involved in" an event or a matter.

関係がない

The negative says something has nothing to do with something else. In speech, が usually turns into は or drops out: かんけいない関係ない.

Usage notes

These go beyond saying that two things are connected.

Before a noun

Inside a clause that describes a noun, が often becomes の: かんけい関係のある, かんけい関係のない.

How close the connection is

ぶかい深い or おおきな大きな in front of 関係 says how strong the link is. なんか何か or なん何らかの leaves it vague.