In SQL Server, joins are case-insensitive. Case-sensitive collations are not supported with ArcGIS.

Is Join case-sensitive MySQL?

In MySQL and a few other database management systems, the default collation is case-insensitive. If, however, you plan on only performing case-sensitive queries on A and B, it may be in your interest to set the default collation on those two tables to case-sensitive.

How do you determine case-sensitive?

SQL Server is, by default, case insensitive; however, it is possible to create a case-sensitive SQL Server database and even to make specific table columns case sensitive. The way to determine if a database or database object is to check its "COLLATION" property and look for "CI" or "CS" in the result.