上題：html
In this tutorial you will create a stored procedure and triggers to check a complex constraint. Consider the table definition below:sql
While the unique constraints defined here are sensible, they are not sufficient to express the constraint that a car (identified by its plate) cannot be rented more than once on any particular day.express
1. Explain briefly why this constraint cannot be expressed using a CHECK constraint.yii
2. Write a stored procedure (function in postgres) which takes as input a plate, start date and end date and throws an error if table car rental contains any row with the same plate where the rental period is different from but overlaps with the given period. Note: Throwing errors in postgres is documented here: https://www.postgresql.org/docs/9.6/static/plpgsql-errors-and-messages.html Note: By checking only for different rental periods we don’t need to worry about the newly inserted or updated tuple using the procedure in a trigger (see next question). Duplicate periods are already prevented by the uniqueness constraints specified.ide
3.Create triggers which invoke the stored procedure created to enforce the 「no duplicate rentals」 constraint. Note: In postgres, trigger functions have no arguments and return type trigger: https://www.postgresql.org/docs/9.6/static/plpgsql-trigger.html Create a separate trigger function that meets these requirements and invokes the function defined earlier.函數
Note: You can test that your triggers work by inserting and updating tuples in car rentals:post
-- should work
INSERT INTO car_rentals VALUES ('2-F4ST', '2015-02-02', '2015-02-11', 'DI123');
UPDATE car_rentals SET start_date = '2015-02-01', end_date = '2015-02-10';
INSERT INTO car_rentals VALUES ('SP33DY', '2015-01-20', '2015-02-05', 'DI234');
-- should fail (test individually)
UPDATE car_rentals SET plate = '2-F4ST' WHERE plate = 'SP33DY';
INSERT INTO car_rentals VALUES ('2-F4ST', '2015-02-10', '2015-02-15', 'DI234');
INSERT INTO car_rentals VALUES ('2-F4ST', '2015-01-20', '2015-02-15', 'DI234');
INSERT INTO car_rentals VALUES ('2-F4ST', '2015-02-02', '2015-02-09', 'DI234');
-- should work
INSERT INTO car_rentals VALUES ('2-F4ST', '2015-03-01', '2015-03-10', 'DI234');測試
-- additional test
update car_rentals set end_date = '2015-03-02' where plate = '2-F4ST' and license_nr='DI234'ui
第一問：check只能檢測固定數值（給出來的值）進行約束，因此布恩可以用check，用觸發器this
首先是觸發器，基本概念和用法：https://www.yiibai.com/postgresql/postgresql-trigger.html
創建一個觸發器，首先先創造出對應函數，即你想讓觸發器完成什麼樣的工做，裏面必需要有對條件的正確判斷，begin開始，end結尾，外層套create的大套子，二三問和在一塊兒寫個觸發器：
1 create or replace function chk_car_rentals() 2 returns trigger as $$ 3 begin 4 if TG_OP='INSERT' then 5 if (select count(*) from car_rentals 6 where new.plate=plate and new.start_date <= end_date and new.end_date >= start_date 7 group by plate) >0 then 8 raise exception 'aaaaa'; 9 return null; 10 end if; 11 elsif TG_OP='UPDATE' then 12 if new.plate <> old.plate then 13 if (select count(*) from car_rentals 14 where new.plate=plate and new.start_date <= end_date and new.end_date >= start_date 15 group by plate) >0 then 16 raise exception 'aaaaa'; 17 return null; 18 end if; 19 elsif new.start_date < old.start_date or new.end_date > old.end_date then 20 if (select count(*) 21 from (select * from car_rentals 22 except 23 select *from car_rentals where old.plate=plate and old.start_date=start_date and old.end_date=end_date and old.license_nr=license_nr) as new_car_rentals 24 where new.plate=plate and new.start_date <= end_date and new.end_date >= start_date 25 group by plate) >0 then 26 raise exception 'bbbb'; 27 return null; 28 end if; 29 end if; 30 end if; 31 return new; 32 end; 33 $$ language plpgsql; 34 35 create trigger trg_car_rental 36 before insert or update on car_rentals 37 for each row execute procedure chk_car_rentals();
--若是是Insert 語句，語句應該是插入的新日期在原日期以內且車牌號相等，致使插入的時間段重複，引起異常
--若是是update 語句，當車牌號發生改變後，觸發觸發器的條件和上面 insert的條件相同，重複將異常 當車牌號不變，利用except除去表中舊記錄的部分，select新的在所剩的表裏面看是否還存在着「撞車」的狀況，把異常條件寫出來就行了接下來就是測試這幾條insert，update語句，應和預期結果相同