Why does the attribute table have null or empty values after a join?

After performing a join between a feature class and a stand-alone table, such as dBASE IV, dBASE V, CSV, or PRN, the values in the fields from the joined table are empty or null. Null values can be the result of several factors:

• Values in the specified fields for the join do not match. Joins are also case sensitive. For example, 'NEW YORK' does not join with 'New York'. To convert string values to the proper case, refer to How To: Convert a string to proper case in the ArcMap Field Calculator for more information.
• The name of the table or feature class, or field names in the table or feature class include spaces or special characters. Refer to FAQ: What characters should not be used in ArcGIS for field names and table names? for more information.
• The table is stored in a Microsoft Access database that is not a personal geodatabase. Refer to How To: Use Microsoft Access tables in ArcGIS for more information.
• The field names in the table are Microsoft Access reserved words. For a list of reserved words, refer to Microsoft Support: List of reserved words in Access 2002 and in later versions of Access for more information.
• The join fields in the two tables are not of the same data type.