A column select expression is a select expression that specifies
    a one-column result table in one row and can be nested
    within predicates and (if they specify a single value) value
    expressions. Column select expressions cannot specify a list of
    select items. You can only specify one value in a select list.

    Column select expressions are also called scalar expressions.

    SQL accepts column select expressions as arguments to IN and
    quantified predicates, and more generally as value expressions.

    o  As arguments to IN and quantified predicates, column select
       expressions specify a collection of values to which SQL
       compares a value expression. Therefore, column select
       expressions as arguments to those predicates can return one
       or more values.

    o  As a type of value expression, column select expressions
       specify a single value. Therefore, a column select expression
       used as a value expression should not return more than one
       value. If it does, SQL generates the following error:

       %RDB-E-MULTIPLE_MATCH, record selection criteria should identify
                              only one record; more than one record found

       If a column select expression used as a value expression
       returns zero rows, SQL evaluates the value expression as null.
       The data type of a column select expression used as a value
       expression is the same as the data type of the column select
       expression's select item.