表架構困難查詢

Question

我工作的一個實踐問題，DDL如下：

CREATE TABLE people (
    id SMALLINT NOT NULL AUTO_INCREMENT, 
    first_name VARCHAR(50), 
    last_name VARCHAR(50), 
    PRIMARY KEY (id) 
) 
; 

CREATE TABLE cd (
    id SMALLINT NOT NULL AUTO_INCREMENT, 
    artist VARCHAR(50), 
    title VARCHAR(50), 
    PRIMARY KEY(id), 
    owner SMALLINT, 
    FOREIGN KEY (owner) REFERENCES people(id) 
) 
; 

CREATE TABLE lend (
    id SMALLINT NOT NULL AUTO_INCREMENT, 
    cd_id SMALLINT, 
    lend_to SMALLINT, 
    FOREIGN KEY (lend_to) REFERENCES people(id), 
    FOREIGN KEY (cd_id) REFERENCES cd(id), 
    lend_date DATE DEFAULT '0000-00-00', 
    PRIMARY KEY(id) 
) 
; 

INSERT INTO people (id, first_name, last_name) VALUES 
(1, 'Brett', 'CEO'), 
(2, 'Jeff', 'President'), 
(3, 'Beta', 'Media'), 
(4, 'Casey', 'Content') 
; 

INSERT INTO cd (id, artist, title, owner) VALUES 
(1, 'The xx', 'Coexist', 2), 
(2, 'ACDC', 'High Voltage', 1), 
(3, 'Bjork', 'Cocoon', 3), 
(4, 'Ella Fitzgerald', 'Ella Sings Gershwin', 4), 
(5, 'Fever Ray', 'Live in Lulea', 2), 
(6, 'Tom Waits', 'Rain Dogs', 4), 
(7, 'Howlin Wolf', 'Smokestack Lightning', 1), 
(8, 'Tupac', 'Poetic Justice', 4) 

; 


INSERT INTO lend (id, cd_id, lend_to, lend_date) VALUES 
(1, 2, 3, '2014/01/03'), 
(2, 3, 1, '2014/04/02'), 
(3, 7, 4, '2013/12/22'), 
(4, 4, 2, '2014/01/03') 

; 

我希望我的查詢，以顯示誰的CD借給。我可以從借出表中獲取ID，但想要顯示從people表中借出的個人的全名。我是否需要重新設計借出表如何連接到人員表，或只是在查詢中使用某種情況下的功能？以下是我的查詢，我得到了l.lent_to並且想要顯示CD借給的CONCAT（p.first_name，''，p.last_name）。

SELECT /*cd.id,*/ 
     CONCAT(p.first_name, ' ', p.last_name) 'CD OWNER', 
     cd.title, 

    l.lend_to, 
    p.id , 

     (
     CASE 
      WHEN l.lend_to IS NULL 
      THEN 'Not Lent' 
      ELSE DATE_FORMAT(l.lend_date, '%m-%d-%Y') 
     END 
    ) 'LEND DATE', 

     (
     CASE 
      WHEN l.lend_to IS NULL 
      THEN 'Not Lent' 
      ELSE TIMESTAMPDIFF(day, l.lend_date, NOW()) 
     END 
     ) 'DAYS LENT' 


FROM 
people p 
LEFT JOIN cd cd 
ON p.id = cd.owner 

LEFT JOIN lend l 
ON cd.id = l.cd_id 

LEFT JOIN lend l1 
on p.id = l1.lend_to 

; 

Answer 1

看看這個查詢爲您提供您正在尋找

select c.title as 'Title', c.artist as 'Artist', o.first_name as 'Owner', 
l.lend_date as 'Lend Date', p.first_name as 'Lender' 
from cd c 
left outer join people o on c.owner = o.id 
left outer join lend l on c.id = l.cd_id 
left outer join people p on l.lend_to = p.id 

的基本信息，您可以添加額外的開關邏輯來提高的結果，如果這是你在找什麼。

Answer 2

我已經解決了數據架構重新設計的問題。如果感興趣，請看一下。

http://sqlfiddle.com/#!2/b6158/3