从表中获取记录的 SQL 查询

Question

我有一个小问题要回答。 有4个表称它为

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

我想显示来自产品表的记录，但要求是 pid（来自产品）和 uid（来自用户）不应出现在同一行的组装表中。

Answer 1

我相信这就是你要找的东西：

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 和其中的 cmets。您在这里涵盖了很多概念，并且您需要考虑您的设计以及您要完成的工作，但要解决您在问题中提出的目标：

1. 您可以使用PRIMARY KEYS 或UNIQUE CONSTRAINTS 来确保不会将重复值插入到您的表中（例如，您不希望pid 和cid 在汇编表中重复，所以我把它作为主键）。我对你的陈述非常直截了当：

要求是 pid（来自产品）和 uid（来自用户）不应出现在同一行的组装表中。 _{^{（强调我的）}}

1. 您可以使用JOINS 将不同表中的项目相互关联。事实上，您的组装表通常被称为“连接表”。我的示例是使用内部连接（意思是两个表中都必须存在值）。

2. 您可以使用FOREIGN KEYS 来确保您要在另一个表中引用的列存在。

下面是 SQL Fiddle 行为不端的代码。

创建和填充表格：

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')
   );

查询：

-- get the data out of your assemble table, with names from related tables
SELECT u.uname, c.cname, p.pname
FROM users u
INNER JOIN assemble j
   ON u.uid = j.uid
INNER JOIN categories c
   ON j.cid = c.cid
INNER JOIN products p
   ON p.pid = j.pid;

-- Give Mom the same coat... this will fail due to the PRIMARY KEY, even with a different category
INSERT INTO assemble (uid, cid, pid)
   VALUES (
     (SELECT uid FROM users WHERE uname='Mom'),
     (SELECT cid FROM categories WHERE cname='Hats'),
     (SELECT pid FROM products WHERE pname='Flying Jacket')
   );