Storing multiple customers

A many-to-many relationship between tasks and customers is usually modeled with an association table, where the primary key is a composite key consisting of the PK from both tables (identifying relationship).

Below is an example including the customers table you presumably have. I suggest using nvarchar instead of nchar for variable-length character data.

CREATE TABLE dbo.tblTask( TaskID int IDENTITY NOT NULL CONSTRAINT PK_tblTask PRIMARY KEY CLUSTERED , TaskName nvarchar(100) , TaskDescription nvarchar(255) ); CREATE TABLE dbo.tblCustomer( CustomerID int IDENTITY NOT NULL CONSTRAINT PK_tblCustomer PRIMARY KEY CLUSTERED , CustomerName nvarchar(100) ); CREATE TABLE dbo.tblTaskCustomer( TaskID int NOT NULL CONSTRAINT FK_tblTaskCustomer_tblTask FOREIGN KEY REFERENCES dbo.tblTask(TaskID) , CustomerID int NOT NULL CONSTRAINT FK_tblTaskCustomer_tblCustomer FOREIGN KEY REFERENCES dbo.tblCustomer(CustomerID) --include other attributes relevant for this customer's association with the task , CONSTRAINT PK_tblTaskCustomer PRIMARY KEY CLUSTERED (TaskID, CustomerID) ); 

If you often query tasks associated with customers, you probably want a non-clustered index (or unique constraint) on CustomerID and TaskID (in that order) to help optimize those queries.

CREATE UNIQUE NONCLUSTERED INDEX idx_tblTaskCustomer_CustomerID_TaskID ON dbo.tblTaskCustomer(CustomerID, TaskID);