Question

我有一个小问题需要回答。有4个表称为

user : uid, uname
category: cid,canme
product : pid, pname
assemble : aid, uid,pid,cid;

我想显示产品表中的记录，但需求是pid（来自产品），而uid（来自用户）不应该一次出现在同一行的汇编表中。

Answer 1

我相信这是你正在寻找的：

select * from user u, product p, assemble a where p.pid not in 
(Select pid from assemble) and u.uid not in (select uid from assemble)

Answer 2

查看this SQL Fiddle及其中的评论。您在这里涵盖了很多概念，并且您想要考虑您的设计以及您想要完成的任务，但要解决您在问题中提出的目标：

您可以使用PRIMARY KEYS或UNIQUE CONSTRAINTS来确保重复的值不会插入到您的表格中（例如，您不想要pid和{{ 1}}要在汇编表中复制，所以我把它作为主键）。我的陈述非常直白：

需求是pid（来自产品）和uid（来自用户） 不应该出现 一次在同一行的汇编表中。 _{^{（强调我的）}}

您可以使用JOINS将不同表格中的项目相互关联。实际上，汇编表通常被称为＆＃34;连接表＆＃34;。我的例子是使用内连接（意思是，值必须存在于两个表中）。
您可以使用FOREIGN KEYS确保要在其他表中引用的列存在。

以下是SQL Fiddle行为不端的代码。

创建并填充表格：

cid

查询：

CREATE TABLE users (
  uid serial,
  uname text,
  CONSTRAINT user_pkey PRIMARY KEY (uid)
  );

CREATE TABLE categories (
  cid serial,
  cname text,
  CONSTRAINT cat_pkey PRIMARY KEY (cid)
  );

 CREATE TABLE products (
   pid serial,
   pname text,
  CONSTRAINT prod_pkey PRIMARY KEY (pid)
   );

 CREATE TABLE assemble (
   join_id serial,
   uid bigint,
   pid bigint,
   cid bigint,
   CONSTRAINT assemble_pkey PRIMARY KEY (uid, pid),
   CONSTRAINT assemble_user_fkey FOREIGN KEY (uid)
      REFERENCES users (uid),
   CONSTRAINT assemble_prod_fkey FOREIGN KEY (pid)
      REFERENCES products (pid),
   CONSTRAINT assemble_cat_fkey FOREIGN KEY (cid)
      REFERENCES categories (cid)
   );

INSERT INTO users (uname) VALUES ('Akshay');
INSERT INTO users (uname) VALUES ('Mom');
INSERT INTO users (uname) VALUES ('Dad');

INSERT INTO categories (cname) VALUES ('Shoes');
INSERT INTO categories (cname) VALUES ('Hats');
INSERT INTO categories (cname) VALUES ('Coats');

INSERT INTO products (pname) VALUES ('Blue Suede Nikes');
INSERT INTO products (pname) VALUES ('Fedora');
INSERT INTO products (pname) VALUES ('Flying Jacket');

-- Give Akshay a hat
INSERT INTO assemble (uid, cid, pid)
   VALUES (
     (SELECT uid FROM users WHERE uname='Akshay'),
     (SELECT cid FROM categories WHERE cname='Hats'),
     (SELECT pid FROM products WHERE pname='Fedora')
   );

-- Give Mom a coat
INSERT INTO assemble (uid, cid, pid)
   VALUES (
     (SELECT uid FROM users WHERE uname='Mom'),
     (SELECT cid FROM categories WHERE cname='Coats'),
     (SELECT pid FROM products WHERE pname='Flying Jacket')
   );

-- Give Dad some shoes
INSERT INTO assemble (uid, cid, pid)
   VALUES (
     (SELECT uid FROM users WHERE uname='Dad'),
     (SELECT cid FROM categories WHERE cname='Shoes'),
     (SELECT pid FROM products WHERE pname='Blue Suede Nikes')
   );

用于从表中获取记录的SQL查询

2 个答案: