PostgreSQL中的约束和断言

Question

我想创建一个简单的数据库，其中有一张客户数据表和一张订单数据表。我正在尝试编写一个约束条件，以便客户在特定日期内不能订购超过特定数量的商品。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 
); 

这是我写的强制执行此约束，但它不工作。我假设它是因为ORDER_NUM和ORDER_DATE从不具有相等的值。

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 

我的问题是如何让这个约束工作，就像我如何限制每天的订单数量。

Answer 1

由于@ruakh已经清理，还有在PostgreSQL里没有CREATE ASSERTION 。只需检查list of SQL commands。它不在那里。

您可以使用触发器更新每个客户的计数以及CHECK约束条件，但必须涵盖所有相关的DML语句：INSERT，UPDATE，DELETE。看起来是这样的：

准备现有的客户表：

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); 

创建触发器功能和触发器：

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触发器，所有这些触发器 - 这就是为什么它是确定RETURN NULL。在这种情况下，AFTER优于BEFORE。如果任何其他条件可以取消中间的DML语句（与其他触发器一样），则性能会更好。

如果你什么也没有，那么BEFORE触发器可能会更好。在这种情况下，请务必使触发功能RETURN NEW/OLD相应。

Answer 2

我不相信PostgreSQL强制执行CREATE ASSERTION语句;至少，“断言”在Appendix D.2 of the PostgreSQL Manual中列为不受支持的功能。据我所知，实际上，主要DBMS的无强制执行它们。

解决方法是使用trigger代替;您可以将其设置为在任何插入ORDERS之前运行，并在检测到此问题时引发错误。 （我假设更新上ORDERS绝不会介绍这个问题，但如果他们能，那么你就需要对这种情况下的触发也。）