Using Index for joining two tables when FK field is NULL - sql

Question

We started to maintain a new project that uses SQL Server and there are some problems regarding to the data in some tables as explained below. There are 2 tables called Employee and Pass and the Pass table keeps the in and out of every Employee.
Employee
|| ID || Name || CardNo ||
======================================
|| 1 || John || 101 ||
|| 2 || Christof || 102 ||
|| 3 || Jurgen || 103 ||
|| 4 || Jose || 104 ||
|| 5 || Mary || 105 ||
Pass
|| ID || EmployeeID || CardNo ||
======================================
|| 1 || NULL || 101 ||
|| 2 || NULL || 105 ||
|| 3 || NULL || 103 ||
|| 4 || NULL || 101 ||
|| 5 || NULL || 102 ||
|| 6 || NULL || 104 ||
|| 7 || NULL || 104 ||
|| 8 || NULL || 103 ||
|| 9 || NULL || 105 ||
|| 10 || NULL || 101 ||
On the other hand, as the EmployeeID column of the Pass table is empty for every pass, we have to use CardNo column in order to join two tables.
But using the JOIN clause as shown below causes the query to be executed more time and I think there might be a better way by using index, etc. instead of PK to optimize the query. We have tried to create index, but could not create for CardNo column.
Could you please clarify us on how to fix the problem? Thanks...
SELECT *
FROM Pass p
LEFT JOIN Employee e ON RIGHT(e.CardNo, 8) = RIGHT(p.CardNo, 8) --I have to trim card no as the digit sizes are different
Update :
I tried to apply the following scripts, but only this part is worked:
alter table Pass add cardno8 as RIGHT(CardNo, 8);
alter table Employee add cardno8 as RIGHT(CardNo, 8);
And this part gives error: "Column 'cardno8' in table 'Pass' is of a type that is invalid for use as a key column in an index."
create index idx_tEvent_cardno8 ON Pass(cardno8);
create index idx_tEmployee_cardno8 ON Employee (cardno8);
Any idea?

Answer 1

If the problem is the performance of the query, then you simply need an index on Employee(CardNo):
create index idx_employee_cardno on employee(cardno);
If you are interested in only one or two other columns (such as name or id), you can add them to the index after cardno.
The only reason that I can think of (apart from permissions) to explain why index creation would fail is that cardno has the wrong type. To quote the documentation:
Columns that are of the large object (LOB) data types ntext, text,
varchar(max), nvarchar(max), varbinary(max), xml, or image cannot be
specified as key columns for an index.
That seems unlikely given the sample data. If this is an issue, then change the type to something more appropriate (or use a computed column for the same purpose).
Also, your query should use qualified column names everywhere (or you'll get an error):
SELECT *
FROM Pass p LEFT JOIN
Employee e
ON e.CardNo = p.CardNo;
--------^