1. mysql keyword---> exists

After the exists is used in the WHERE keyword, the following conditional statement must be a query statement and return a Boolean type.
For example:
Select from a where exists (select b_id from b where b.b_name = A.a_name)
When the conditional statement select b_id from b where b.b_name = A.a_name has a value, it is true, which is equivalent to executing the part of the conditional statement where it is later. Then the entire SQL statement is equivalent to:
Select from a where a_name in (select B_name from B)

1. mysql keyword---> exists