The SINGLE predicate tests whether or not the result table
    specified in the column select expression has exactly one row.
    If it has exactly one row, SQL evaluates the SINGLE predicate as
    true. If the result table has zero rows or more than one row, the
    predicate is false.

    SQL evaluates the NOT SINGLE predicate as true if the result
    table specified in the select expression has zero rows or more
    than one row.

    The SINGLE and NOT SINGLE predicates cannot be unknown.

    The SINGLE predicate has the following form:

  single-predicate =

  ---> SINGLE --> ( select-expr ) ----->

    Because it checks only for the existence of rows, a SINGLE
    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
    SINGLE predicates, an asterisk (*)  wildcard in the column select
    expression can refer to a multicolumn table (as in the following
    example).