ALTER TABLE [dbo].[HelpDeskCategoryThemeTaskTypes]
ALTER COLUMN [hdcttt_theme_id] UNIQUEIDENTIFIER NULL 

CREATE TABLE [dbo].HelpDeskCategoryThemeTaskTypes (
    [hdcttt_id]          UNIQUEIDENTIFIER DEFAULT (NEWID()) NOT NULL,
    [hdcttt_theme_id]    UNIQUEIDENTIFIER NULL FOREIGN KEY REFERENCES HelpDeskCategoryTheme([hdct_id]),
    [hdcttt_org_id]      UNIQUEIDENTIFIER NULL FOREIGN KEY REFERENCES Orgs(org_id),
    [hdcttt_system_id]   UNIQUEIDENTIFIER NULL FOREIGN KEY REFERENCES TaskParamValues([tpval_id]),
    [hdcttt_ttype_id]    UNIQUEIDENTIFIER NOT NULL FOREIGN KEY REFERENCES TaskTypes([ttype_id]),
    [hdcttt_priority]    INT              NOT NULL DEFAULT(0),
    [fl_deleted]         DATETIME         NULL,
    [fl_created]         DATETIME         NOT NULL DEFAULT(GETDATE()),
    PRIMARY KEY CLUSTERED ([hdcttt_id] ASC)
);