Developing Portable Applications : 3. Application Considerations : Data Type Limits : How Empty Strings are Processed
 
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How Empty Strings are Processed
Ingres allows empty strings such as '' (also known as zero-length strings) to be stored in character data types, which is not allowed by Oracle (see Note). Applications such as OpenROAD rely on the ability to insert a zero-length character into a char or varchar field.
Note:  Oracle treats empty strings ('') as NULL.
For Oracle and Microsoft SQL Server, the gateway handles empty strings as follows:
If an empty string is received from the client, the gateway stores it as a single blank character.
When data is retrieved, if the data type is character and if it contains a single blank character, the gateway returns an empty string to the client.
A zero-length character and a one-byte character that contains a single blank character appear to be treated identically.
Note:  Microsoft SQL Server 7.0 added support for empty strings. However, the gateway empty string processing has not changed.