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.

1  –  Environment

    You can use column select expressions in interactive SQL or in
    host language programs.

2  –  Format

  col-select-expr =

  ---> select-expr ------->

  select-expr =

  -+-+->  select-clause  ------------+-+------+
   | +-> ( select-expr )  -----------+ |      |
   | +->   TABLE table-ref ----------+ |      |
   +------ select-merge-clause <-------+      |
     +------------------- <-------------------+
     +-+--------------------+--+------------------+--+--------------------+->
       +-> order-by-clause -+  +-> offset-clause -+  +-> limit-to-clause -+

3  –  Arguments

3.1  –  select-expr

    A column select expression is a select expression specifying only
    one value in the select list. See Select_Expressions for more
    information.