phpMyAdmin - SQL查詢

Question

此文件顯示客戶數據庫和CD數據庫。
客戶被外鍵爲buyerIDs。一些客戶/買家已經購買了一張特定CD的多份副本。
我創建在記事本文件++，導入到phpMyAdmin的，我打得四處查詢部分來嘗試獲得的例子聲明：

Customer Gerald Bostick bought 3 copies of Thick as a Brick 

我想出了查詢：

SELECT `customer`.`CustName`, `customer`.`CDPurchases`, `cd`.`Title` 
FROM `customer`, `cd` 

我得到的是表中的每一個購買CD一位客戶：

Joe Doe 12 Ascension 
Suzy Creamcheese 3 Ascension 
Jane Doe 1 Ascension 
Gerald Bostick 3 Ascension 
Lisa Simpson NULL Ascension 
Joe Doe 12 The Velvet Rope 
Suzy Creamcheese 3 The Velvet Rope 
Jane Doe 1 The Velvet Rope 
Gerald Bostick 3 The Velvet Rope 
Lisa Simpson NULL The Velvet Rope 
Joe Doe 12 The Pecan Tree 
Suzy Creamcheese 3 The Pecan Tree 
Jane Doe 1 The Pecan Tree 
Gerald Bostick 3 The Pecan Tree 
Lisa Simpson NULL The Pecan Tree 

是外鍵設置是否正確？我改變了它從

CONSTRAINT FK_Buyer FOREIGN KEY FK_Buyer (BuyerId) 
REFERENCES Customer (CustID) 

到

CONSTRAINT FK_Buyer FOREIGN KEY (BuyerId) 
REFERENCES Customer (CustID) 

或者是SET陳述或我的查詢不正確的？

我的編碼是：

DROP DATABASE IF EXISTS Library; 
CREATE DATABASE Library; 
USE Library; 
DROP TABLE IF EXISTS Customer; 
DROP TABLE IF EXISTS CD; 


CREATE TABLE Customer (
    CustID INTEGER UNSIGNED NOT NULL AUTO_INCREMENT, 
    CustName VARCHAR(20) NOT NULL, 
    CDPurchases INTEGER, 
    PRIMARY KEY (CustID) 
); 

CREATE TABLE CD (
    CDID INTEGER UNSIGNED NOT NULL AUTO_INCREMENT, 
    Title VARCHAR(45) NOT NULL, 
    BuyerId INTEGER UNSIGNED, 
    Price FLOAT(6,2) UNSIGNED NOT NULL, 
    PRIMARY KEY (CDID), 
    CONSTRAINT FK_Buyer FOREIGN KEY (BuyerId) 
    REFERENCES Customer (CustID) 
    ON DELETE RESTRICT 
    ON UPDATE RESTRICT 
); 

INSERT INTO Customer VALUES (null, "Joe Doe", 12); 
SET @joedoe := LAST_INSERT_ID(); 
INSERT INTO Customer VALUES (null, "Suzy Creamcheese", 3); 
INSERT INTO Customer VALUES (null, "Jane Doe", 1); 
SET @janedoe := LAST_INSERT_ID(); 
INSERT INTO Customer VALUES (null, "Gerald Bostick",3); 
SET @geraldbostick := LAST_INSERT_ID(); 
INSERT INTO Customer VALUES (null, "Lisa Simpson", null); 

INSERT INTO CD VALUES (null, "Thriller", @janedoe, 12.99); 
INSERT INTO CD VALUES (null, "Grown and Sexy", null, 16.95); 
INSERT INTO CD VALUES (null, "Ascension", null, 14.50); 
INSERT INTO CD VALUES (null, "The Velvet Rope", null, 13.85); 
INSERT INTO CD VALUES (null, "The Pecan Tree", null, 9.99); 
INSERT INTO CD VALUES (null, "Condensate", null, 11.85); 
INSERT INTO CD VALUES (null, "The Dana Owens Album", null, 9.95); 
INSERT INTO CD VALUES (null, "Dustbowl", @joedoe, 17.95); 
INSERT INTO CD VALUES (null, "Thick as a Brick", @geraldbostick, 10.95); 

感謝您的任何幫助。

Answer 1

這是因爲你的查詢創建一個交叉聯接或笛卡爾積（X * X）

SELECT customer.CustName, customer.CDPurchases, cd.Title 
FROM customer, cd 
WHERE customer.custid = cd.buyerid; 

當您加入兩個表一起，數據庫引擎不承擔您的加盟條件 - 你必須明確說明它。有外鍵約束的事實只能提供更好的引用完整性 - 它不會強制表加入。