PostgreSQL中的約束和斷言

Question

我想創建一個簡單的數據庫，其中有一張客戶數據表和一張訂單數據表。我正在嘗試編寫一個約束條件，以便客戶在特定日期內不能訂購超過特定數量的商品。下面是我有：

CREATE TABLE CUSTOMER 
(
    CUSTOMER_NUM CHAR(3) PRIMARY KEY, 
    CUSTOMER_NAME CHAR(35) NOT NULL, 
    STREET CHAR(15), 
    CITY CHAR(15), 
    STATE CHAR(3), 
    ZIP CHAR(5), 
); 
CREATE TABLE ORDERS 
(
    ORDER_NUM CHAR(5) PRIMARY KEY, 
    ORDER_DATE DATE, 
    CUSTOMER_NUM CHAR(3), 

    CONSTRAINT CUSTOMER_NUM_FKEY FOREIGN KEY (CUSTOMER_NUM) 
     REFRENCES CUSTOMER (CUSTOMER_NUM) MATCH SIMPLE 
     ON UPDATE CASCADE ON DELETE CASCADE 
); 

這是我寫的強制執行此約束，但它不工作。我假設它是因爲ORDER_NUM和ORDER_DATE從不具有相等的值。

CREATE ASSERTION ITEM_LIMIT 
CEHCK(
     ( SELECT COUNT(*) 
      FROM CUSTOMER C1, ORDERS O1 
      WHERE C1.CUSTOMER_NUM = O1.CUSTOMER_NUM AND 
       O1.ORDER_DATE = O1.ORDER_NUM 
    ) <= 1000 

我的問題是如何讓這個約束工作，就像我如何限制每天的訂單數量。

Answer 1

由於@ruakh已經清理，還有在PostgreSQL裏沒有CREATE ASSERTION 。只需檢查list of SQL commands。它不在那裏。

您可以使用觸發器更新每個客戶的計數以及CHECK約束條件，但必須涵蓋所有相關的DML語句：INSERT，UPDATE，DELETE。看起來是這樣的：

準備現有的客戶表：

ALTER TABLE customer ADD COLUMN order_ct integer DEFAULT 0; 
UPDATE customer SET order_ct = 0; 
ALTER TABLE customer ALTER order_ct SET NOT NULL; 
ALTER TABLE customer ADD CONSTRAINT order_ct_max1000 CHECK (order_ct <= 1000); 

創建觸發器功能和觸發器：

CREATE OR REPLACE FUNCTION trg_order_upaft() 
    RETURNS trigger AS 
$BODY$ 
BEGIN 

IF OLD.customer_num <> NEW.customer_num THEN 
    UPDATE customer 
    SET order_ct = order_ct - 1 
    WHERE customer_num = OLD.customer_num; 

    UPDATE customer 
    SET order_ct = order_ct + 1 
    WHERE customer_num = NEW.customer_num; 
END IF; 

RETURN NULL; 

END; 
$BODY$ 
    LANGUAGE plpgsql; 

CREATE TRIGGER upaft 
    AFTER UPDATE ON orders FOR EACH ROW 
    EXECUTE PROCEDURE trg_order_upaft(); 


CREATE OR REPLACE FUNCTION trg_order_insaft() 
    RETURNS trigger AS 
$BODY$ 
BEGIN 

UPDATE customer 
SET order_ct = order_ct + 1 
WHERE customer_num = NEW.customer_num; 

RETURN NULL; 

END; 
$BODY$ 
    LANGUAGE plpgsql; 

CREATE TRIGGER insaft 
    AFTER INSERT ON orders FOR EACH ROW 
    EXECUTE PROCEDURE trg_order_insaft(); 


CREATE OR REPLACE FUNCTION trg_order_delaft() 
    RETURNS trigger AS 
$BODY$ 
BEGIN 

UPDATE customer 
SET order_ct = order_ct - 1; 
WHERE customer_num = OLD.customer_num; 

RETURN NULL; 

END; 
$BODY$ 
    LANGUAGE plpgsql; 

CREATE TRIGGER delaft 
    AFTER DELETE ON orders FOR EACH ROW 
    EXECUTE PROCEDURE trg_order_delaft(); 

我做了AFTER觸發器，所有這些觸發器 - 這就是爲什麼它是確定RETURN NULL。在這種情況下，AFTER優於BEFORE。如果任何其他條件可以取消中間的DML語句（與其他觸發器一樣），則性能會更好。

如果你什麼也沒有，那麼BEFORE觸發器可能會更好。在這種情況下，請務必使觸發功能RETURN NEW/OLD相應。

Answer 2

我不相信PostgreSQL強制執行CREATE ASSERTION語句;至少，「斷言」在Appendix D.2 of the PostgreSQL Manual中列爲不受支持的功能。據我所知，實際上，主要DBMS的無強制執行它們。

解決方法是使用trigger代替;您可以將其設置爲在任何插入ORDERS之前運行，並在檢測到此問題時引發錯誤。 （我假設更新上ORDERS絕不會介紹這個問題，但如果他們能，那麼你就需要對這種情況下的觸發也。）