SQL 语句。需要帮助理解它

Question

我发现很难理解这段代码的作用。有人可以帮助我逐行理解这段代码，以便我可以理解它在做什么。

CREATE TRIGGER LowCredit ON Order
AFTER INSERT
AS

DECLARE @creditrating tinyint

SELECT @creditrating = v.CreditRating
FROM Customer c INNER JOIN inserted i
ON c.custID = i.custID

IF @creditrating = 5
BEGIN

    RAISERROR ('This customers''s credit rating
    is too low to accept new orders.’)

    ROLLBACK TRANSACTION

END

Answer 1

它正在检查信用评级是否为某个值，如果它太低会引发错误并回滚交易。

--Declare a trigger with name `LowCredit` on table `Order`, 
--run the trigger after 
CREATE TRIGGER LowCredit ON Order  
insert.
AFTER INSERT

AS
--start definition
--declare int
DECLARE @creditrating tinyint

--select from existing customer record the
-- inserted rows credit ranking (by custID)
-- inserted is the vt containing the changed rows
SELECT @creditrating = v.CreditRating
   FROM Customer c INNER JOIN inserted i
ON c.custID = i.custID
--if lower than 5 roll back
IF @creditrating = 5
BEGIN
--raise error to the session
RAISERROR ('This customers''s credit rating
is too low to accept new orders.’)
--roll back transaction
ROLLBACK TRANSACTION

END