An EXISTS predicate tests whether or not the result table
    specified in a column select expression is empty.

  exists-predicate =

  ---> EXISTS ( select-expr ) ------->

    If the result table specified in the select expression has one or
    more rows, SQL evaluates the EXISTS predicate as true. Otherwise,
    the predicate is false. An EXISTS predicate cannot be unknown.

    Because it only checks for the existence of rows, an EXISTS
    predicate does not require that the result table from its column
    select expression be a single column wide (see Column_Select_
    Expressions for details on column select expressions). For
    EXISTS predicates, an asterisk (*)  wildcard in the column select
    expression can refer to a multicolumn table .