deleting an employee conflicts with department manager foreign key constraint - sql-server

Question

I created two tables , departments and employees with the following structure :
CREATE TABLE [dbo].[departments](
[department_id] [bigint] IDENTITY(10,10) NOT NULL,
[department_name] [nvarchar](30) NOT NULL,
[manager_id] [bigint] NULL,
[location_id] [bigint] NULL,
[department_notes] [varchar](150) NULL,
[created_by] [bigint] NULL,
[created_date] [datetime] NULL,
[last_updated_by] [bigint] NULL,
[last_updated_date1] [datetime] NULL,
[status] [varchar](12) NOT NULL,
CONSTRAINT [PK_departments] PRIMARY KEY CLUSTERED
(
[department_id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY],
CONSTRAINT [UQ_NoRepeat] UNIQUE NONCLUSTERED
(
[department_name] ASC,
[location_id] ASC,
[status] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
SET ANSI_PADDING OFF
GO
ALTER TABLE [dbo].[departments] ADD CONSTRAINT [DF_departments_created_date] DEFAULT (getdate()) FOR [created_date]
GO
ALTER TABLE [dbo].[departments] ADD CONSTRAINT [DF_departments_created_date1] DEFAULT (getdate()) FOR [last_updated_date1]
GO
ALTER TABLE [dbo].[departments] ADD CONSTRAINT [DF_departments_status] DEFAULT ('Active') FOR [status]
GO
ALTER TABLE [dbo].[departments] WITH CHECK ADD FOREIGN KEY([manager_id])
REFERENCES [dbo].[employees] ([employee_id])
GO
ALTER TABLE [dbo].[departments] WITH CHECK ADD CONSTRAINT [FK_departments_locations] FOREIGN KEY([location_id])
REFERENCES [dbo].[locations] ([location_id])
GO
ALTER TABLE [dbo].[departments] CHECK CONSTRAINT [FK_departments_locations]
GO
ALTER TABLE [dbo].[departments] WITH CHECK ADD CONSTRAINT [Ck_deptStatus] CHECK (([Status]='Deleted' OR [Status]='Active'))
GO
ALTER TABLE [dbo].[departments] CHECK CONSTRAINT [Ck_deptStatus]
GO
CREATE TABLE [dbo].[employees](
[employee_id] [bigint] NOT NULL,
[first_name] [nvarchar](20) NULL,
[last_name] [nvarchar](25) NOT NULL,
[email] [nvarchar](25) NOT NULL,
[phone_number] [nvarchar](20) NULL,
[hire_date] [date] NULL,
[job_id] [nvarchar](10) NOT NULL,
[salary] [numeric](8, 2) NOT NULL,
[commission_pct] [numeric](2, 2) NULL,
[manager_id] [bigint] NULL,
[department_id] [bigint] NOT NULL,
[allow_login] [bit] NOT NULL,
[user_id] [nvarchar](128) NULL,
[allow_email] [bit] NOT NULL,
[driv_lic_no] [nchar](20) NULL,
[reporting_to] [bigint] NULL,
[salutation] [nchar](10) NULL,
[date_of_birth] [date] NULL,
[gender] [nchar](10) NULL,
[blood_group] [nchar](10) NULL,
[Nationality] [nchar](10) NULL,
[gov_id] [nchar](25) NULL,
[passport_no] [nchar](25) NULL,
[passport_expir] [date] NULL,
[driv_lic_expir] [date] NULL,
[perm_address] [varchar](250) NULL,
[perm_city] [varchar](50) NULL,
[perm_state] [varchar](50) NULL,
[per_zip] [nchar](20) NULL,
[perm_country] [nvarchar](6) NULL,
[current_address] [varchar](250) NULL,
[current_city] [varchar](50) NULL,
[current_state] [varchar](50) NULL,
[current_zip] [nchar](20) NULL,
[current_country] [nvarchar](6) NULL,
[mobile_no] [nvarchar](20) NULL,
[notes] [varchar](250) NULL,
[added_by] [bigint] NULL,
[added_on] [date] NULL,
[send_cred_by_email] [bit] NOT NULL,
[user_name] [nvarchar](256) NULL,
CONSTRAINT [PK_employees] PRIMARY KEY CLUSTERED
(
[employee_id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
SET ANSI_PADDING OFF
GO
ALTER TABLE [dbo].[employees] ADD CONSTRAINT [DF_employees_allow_login] DEFAULT ((0)) FOR [allow_login]
GO
ALTER TABLE [dbo].[employees] ADD CONSTRAINT [DF_employees_allow_email] DEFAULT ((0)) FOR [allow_email]
GO
ALTER TABLE [dbo].[employees] ADD DEFAULT ('Mr.') FOR [salutation]
GO
ALTER TABLE [dbo].[employees] ADD DEFAULT ((1)) FOR [send_cred_by_email]
GO
ALTER TABLE [dbo].[employees] WITH CHECK ADD CONSTRAINT [FK_employees_AspNetUsers] FOREIGN KEY([user_id])
REFERENCES [dbo].[AspNetUsers] ([Id])
GO
ALTER TABLE [dbo].[employees] CHECK CONSTRAINT [FK_employees_AspNetUsers]
GO
ALTER TABLE [dbo].[employees] WITH CHECK ADD CONSTRAINT [FK_employees_countries] FOREIGN KEY([perm_country])
REFERENCES [dbo].[countries] ([country_id])
GO
ALTER TABLE [dbo].[employees] CHECK CONSTRAINT [FK_employees_countries]
GO
ALTER TABLE [dbo].[employees] WITH CHECK ADD CONSTRAINT [FK_employees_countries1] FOREIGN KEY([current_country])
REFERENCES [dbo].[countries] ([country_id])
GO
ALTER TABLE [dbo].[employees] CHECK CONSTRAINT [FK_employees_countries1]
GO
ALTER TABLE [dbo].[employees] WITH CHECK ADD CONSTRAINT [FK_employees_employees] FOREIGN KEY([employee_id])
REFERENCES [dbo].[employees] ([employee_id])
GO
ALTER TABLE [dbo].[employees] CHECK CONSTRAINT [FK_employees_employees]
GO
there were some existing employees which are already managers in departments table , now when I try to delete any of those manager employees, I get a foreign key constraint conflict error blocking the delete statement.
what is the problem please ?

Answer 1

The problem is that you're not allowed to do that.
A foreign key constraint is an integrity constraint meaning that it ensures data integrity across tables.
If one department row says that the ID of the employee that is the manager of this department is 73, it would not be good if there is no employee in the database with an ID of 73.
A foreign key constraint ensures this won't happen:
You cannot insert or update a department row to have a manager id that does not exist in the employee table
You cannot delete an employee that is referenced as a manager from a department row
You cannot update the ID of an employee that is referenced as a manager from a department row
The solution is to first fix the department by:
Switching to a different manager by updating the department row and adjusting the ID of the manager
NULL'ing out the manager, saying that this department has no manager, if that is allowed
After doing any of those two actions you should be able to delete the employee, assuming there's no other foreign key references as well.