nvarchar or varchar or date in joins of SQL Server

Question

I have a management project developed in Web using SQL Server 2008 R2 and Windows Application using SQL Server CE 4.0.

Both uses almost same database structure. I have a fees table as :

TABLE [fees] 
(
    [feeid] bigint NOT NULL IDENTITY(1,1),
    [acno] int NULL DEFAULT 0,
    [billingmonth] nvarchar(10) NULL,
    [oldbal] numeric(18,2) NULL DEFAULT 0,
    [duedtamt] numeric(18,2) NULL DEFAULT 0,
    [afterdtamt] numeric(18,2) NULL DEFAULT 0, 
    [bal] numeric(18,2) NULL DEFAULT 0,
    [depamt] numeric(18,2) NULL DEFAULT 0,
    [totdpo] numeric(18,2) NULL DEFAULT 0,
    [depdt] datetime NULL
)

billingmonth will always use format MMM-yyyy eg. Jan-2018

BillingMonth uses various joins (inner and left outer join) in other tables.

How to increase performance of joins with BillingMonth ? , should i :

• Convert nvarchar to varchar ( as it will always store Month in SQL2008R2 )
• Convert nvarchar to datetime ( as first day of month 01-MMM-yyyy in SQL Server CE and SQL Server 2008 R2)

Answer 1

Storing date values as nvarchar is not recommended at all

There are many suggestions to increase the join performance:

• Use Date datatype
• Use two numeric field month and year instead of one varchar field (tinyint for month, smallint for year, they can be used only for joining purpose)

Note that: AS @Pரதீப் mentioned, When you store month and year separately, you need to do some integer manipulations when searching for date ranges