SQL query if exist and If not exist

Sir/Miss
if exist and if not exist ye vala mujhe samjha nahi. pls explain sir/miss

Suppose you have an :apple:apple, and you ask your friends A :boy: and B :woman:
Do i have an :apple:? (if exist :apple:apple?)
your friend A :boy: replies YES(true boolean)
your friend B :woman: replies NO(false boolean)
But you know the truth answer is YES(true boolean)
Thus in sql
if exist = true returns at lease 1 row
if exist = false returns at no row

Suppose you do not have a football :soccer:, and you ask your friends A :boy: and B :woman:
Do not i have a :soccer:? (if not exist :soccer: football?)
your friend A :boy: replies YES(true boolean) = you have
your friend B :woman: replies NO(false boolean) = you do not have
But you know the truth answer is NO(false boolean)
Thus in sql
if not exist = true returns at no row
if not exist = false returns at lease 1 row

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