How does this create view code work? - sql

Question

Ok, so here is example code for view. Second part with select RT.RoleID[...] seems clear to me. That's the part that's going to be displayed when the query will be run to use the view. first part is unclear though.
First two lines are standard as i understand, then goes the whole with section. Could someone explain it to me? Never seen with "something" as (select) formula
CREATE VIEW [dbo].[sviw_System_MyPermissions_CurrentDomain]
AS
WITH MyDomainRoles AS (
SELECT RM.Domain, RM.RoleID
FROM stbl_System_RolesMembersDomains AS RM WITH (NOLOCK)
WHERE RM.Domain = (SELECT CurrentDomain FROM stbl_System_Users WITH (NOLOCK) WHERE Login = SUSER_SNAME())
AND RM.Login = SUSER_SNAME()
)
SELECT RT.RoleID, RT.TableName, DR.Domain, RT.GrantUpdate, RT.GrantInsert, RT.GrantDelete
FROM stbl_System_RolesTables AS RT WITH (NOLOCK)
JOIN MyDomainRoles AS DR ON RT.RoleID = DR.RoleID
GO

Answer 1

It is called Common Table Expresion and basically your view is the same as:
CREATE VIEW vSalesStaffQuickStats
AS
SELECT E.EmployeeID,
EmployeeOrders = OS.NumberOfOrders,
EmployeeLastOrderDate = OS.MostRecentOrderDate,
E.ManagerID,
ManagerOrders = OM.NumberOfOrders,
ManagerLastOrderDate = OM.MostRecentOrderDate
FROM HumanResources.Employee AS E
INNER JOIN (
SELECT SalesPersonID, COUNT(*) NumberOfOrders
, MAX(OrderDate) MostRecentOrderDate
FROM Sales.SalesOrderHeader
GROUP BY SalesPersonID
) AS OS
ON E.EmployeeID = OS.SalesPersonID
LEFT OUTER JOIN (
SELECT SalesPersonID, COUNT(*) NumberOfOrders
, MAX(OrderDate) MostRecentOrderDate
FROM Sales.SalesOrderHeader
GROUP BY SalesPersonID
) AS OM
ON E.ManagerID = OM.SalesPersonID
As you see you could easily exchange it with subquery. But in your case you need do it twice (with CTE you do it only once).
EDIT:
With new query after update:
CREATE VIEW [dbo].[sviw_System_MyPermissions_CurrentDomain]
AS
SELECT RT.RoleID, RT.TableName, DR.Domain, RT.GrantUpdate, RT.GrantInsert, RT.GrantDelete
FROM stbl_System_RolesTables AS RT WITH (NOLOCK)
JOIN (
SELECT RM.Domain, RM.RoleID
FROM stbl_System_RolesMembersDomains AS RM WITH (NOLOCK)
WHERE RM.Domain = (SELECT CurrentDomain FROM stbl_System_Users WITH (NOLOCK) WHERE Login = SUSER_SNAME())
AND RM.Login = SUSER_SNAME()
) AS DR ON RT.RoleID = DR.RoleID

Answer 2

The WITH keyword just refers to Common Table Expression ( read more here: https://learn.microsoft.com/en-us/sql/t-sql/queries/with-common-table-expression-transact-sql ) Basically everything within the WITH statement could be considered as externalized subquery which result set is then used in the main query.
i.e.:
WITH MyDomainRoles AS (
--- sub-query Start
SELECT RM.Domain, RM.RoleID
FROM stbl_System_RolesMembersDomains AS RM WITH (NOLOCK)
WHERE RM.Domain = (SELECT CurrentDomain FROM stbl_System_Users WITH (NOLOCK) WHERE Login = SUSER_SNAME())
AND RM.Login = SUSER_SNAME()
--- sub-query End
)
... and the result set from the sub-query above is held within the CTE named MyDomainRoles. Then you can refer to MyDomainRoles as to a table. Makes things simpler to read and cleaner to write.
Since this CTE is so simple, you could easily re-write it as follows but it just does not look as neat:
WITH MyDomainRoles AS (
)
SELECT RT.RoleID, RT.TableName, DR.Domain, RT.GrantUpdate, RT.GrantInsert, RT.GrantDelete
FROM stbl_System_RolesTables AS RT WITH (NOLOCK)
JOIN (
SELECT RM.Domain, RM.RoleID
FROM stbl_System_RolesMembersDomains AS RM WITH (NOLOCK)
WHERE RM.Domain = (SELECT CurrentDomain FROM stbl_System_Users WITH (NOLOCK) WHERE Login = SUSER_SNAME())
AND RM.Login = SUSER_SNAME()
) AS DR ON RT.RoleID = DR.RoleID