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The EXISTS operator checks how many records are in the result of a subquery. If it is empty, then the operator returns 0. Otherwise, it returns 1. EXISTS can also be used in a WHERE clause.
References to main query tables and columns are not supported in a subquery.
Syntax 
EXISTS(subquery)
Example Query checking existence of values in a subquery:
Query
SELECT EXISTS(SELECT * FROM numbers(10) WHERE number > 8), EXISTS(SELECT * FROM numbers(10) WHERE number > 11)
Response
┌─in(1, _subquery1)─┬─in(1, _subquery2)─┐
│                 1 │                 0 │
└───────────────────┴───────────────────┘
Query with a subquery returning several rows:
Query
SELECT count() FROM numbers(10) WHERE EXISTS(SELECT number FROM numbers(10) WHERE number > 8);
Response
┌─count()─┐
│      10 │
└─────────┘
Query with a subquery that returns an empty result:
Query
SELECT count() FROM numbers(10) WHERE EXISTS(SELECT number FROM numbers(10) WHERE number > 11);
Response
┌─count()─┐
│       0 │
└─────────┘