OpenEdge Data Management: DataServer for Microsoft SQL Server

Unknown value (?)
Data types
The DataServer supports null values. Procedures that use a null value behave exactly
as they do when accessing an Unknown value (?) in an OpenEdge database, except
for one difference—you cannot compare a field to the Unknown value (?) if the field is
not allowed to hold the Unknown value (?) (i.e., is not null-capable). For example, if the
custnum field is not null-capable, the following statement fails at run time:
FIND customer WHERE customer.custnum NE ?
A column that is not null-capable is marked “mandatory” in the schema holder.
In a DataServer application, you assign the Unknown value (?) to a column by using
the question mark operator (?), which the DataServer translates to the appropriate
null-value representation. For example, the following procedure assigns the Unknown
value (?) to the address2 field of the customer table:
FIND FIRST customer.
customer.address2 = ?.
Zero-length character strings
When you use the Unknown value (?) in a WHERE clause with the DataServer, the
Unknown value (?) satisfies only the equals (=) or not equal () operator. You can also
use a zero-length character string in a WHERE clause. The Unknown value (?) and
zero-length character string are not the same. The Unknown value (?) translates to a
NULL, which is a special marker in a data source that supports NULLs used to represent
missing information. On the other hand, zero-length strings and blank columns contain
actual values. Both of the following statements find the first customer record with a
zero-length string in the address2 field. Notice the space between the quotation marks
in the first statement:
FIND FIRST customer WHERE customer.address2 EQ " ".
FIND FIRST customer WHERE customer.address2 EQ "".
Although “” and “ ” evaluate the same way in a WHERE clause, they have different results
when you use them with the BEGINS function. For example, the following statement
retrieves all customer names except those that have the Unknown value (?):
FOR EACH customer WHERE customer.name BEGINS "":
The following statement uses "" to retrieve only those names that begin with a space:
FOR EACH customer WHERE customer.name BEGINS " ":
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