Subqueries are not allowed in this context. Only scalar expressions are allowed in CREATE TABLE with AS syntax - sql

Question

I am trying to run the below script but I keep getting the error messages
"Subqueries are not allowed in this context. Only scalar expressions are allowed." on lines 16 and 34.
I know where it's failing - it is failing on the AS clauses, but I don't know how to correct it with different code to stop the errors from appearing.
I have tried looking around at other existing questions but none helped me that I can find. As the issue I have here is using data from columns in different tables along with columns in the current table.
Could I get some help with getting this working and advise what code will be better please?
Thanks for your help in advance!
Dan
This is the code for my database::
CREATE DATABASE [LEARNING]
GO
CREATE TABLE Trainees
(
Trainee_ID int IDENTITY(1,1) PRIMARY KEY NOT NULL,
Name varchar(50) NOT NULL,
[Assigned Tutor_ID] int NOT NULL,
)
GO
CREATE TABLE Tutors
(
Tutor_ID int IDENTITY(1,1) PRIMARY KEY NOT NULL,
Name varchar(50) NOT NULL,
[Assigned Trainee_ID] AS (Select Trainee_ID from Trainees where Tutors.[Assigned Trainee_ID] = Trainees.Trainee_ID) NOT NULL
)
GO
CREATE TABLE [Rooms]
(
Room_ID int IDENTITY(1,1) PRIMARY KEY NOT NULL,
[Room Name] varchar(50) NOT NULL,
[Cost per hour] money NOT NULL
)
GO
CREATE TABLE [Rooms Rented]
(
Rented_ID int IDENTITY(1,1) PRIMARY KEY NOT NULL,
Room_ID int NOT NULL,
Tutor_ID int NOT NULL,
[Length of time in hours] int NOT NULL,
[Total Cost] AS (select ([Rooms Rented].[Length of time in hours])*([Rooms].[Cost per hour]) from [Rooms]) NOT NULL
)
GO
INSERT INTO Tutors values ('Nikki Smith',1)
GO
INSERT INTO Trainees Values ('Tyler Hatherall')
GO
INSERT INTO Rooms values ('Training Room 1',6.50)
GO
INSERT INTO [Rooms Rented] values (1,1,2)
GO

Answer 1

Computed columns are used to ensure columns persisted property within the table itself.
In your case, you need to have another update after you created table, populate the column by the query similar to below query, also, you need to create Foreign Key in the Total Cost column based on what you try to achieve.
UPDATE A
SET A.[Total Cost] = A.[Length of time in hours] * B.[Cost per hour] --add ISNULL to treat NULL if needed
FROM [Rooms Rented] as A
INNER JOIN [Rooms] as B
ON B.Room_ID = A.Room_ID

Answer 2

Your AS statements are computed columns. When your computed columns refer to other tables, you cannot implement this directly. You will have to create scalar functions first.
For example, after creating Rooms, create this function that takes a room id and returns cost per hour:
create function f_get_Rooms_CostPerHour (#Room_ID int)
returns money
as
return (select [Cost per hour] from [Rooms] where [Rooms].Room_ID=#Room_ID)
Now you can use this in your computed column formula. Note that a computed column formula never has a SELECT in it. It also does not have a null/not null specification.
CREATE TABLE [Rooms Rented]
(
Rented_ID int IDENTITY(1,1) PRIMARY KEY NOT NULL,
Room_ID int NOT NULL,
Tutor_ID int NOT NULL,
[Length of time in hours] int NOT NULL,
[Total Cost] AS ([Length of time in hours]*f_get_Rooms_CostPerHour([Room_ID]))
)