創建視圖（存在或位於）

Question

創建一個視圖，該視圖包含在Perryridge分支中擁有貸款和帳戶的所有客戶的名稱。

對此的任何想法？我很困惑是否要使用存在或

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的表是：

customer (customer_name, customer_street, customer_city) 
    depositor (customer_name, account_number) 
    account (account_number, branch_name, balance) 
    borrower (customer_name, loan_number) 
    loan (loan_number, branch_name, amount) 

Answer 1

您可以使用一個EXISTS或IN謂詞查詢。要麼會工作。這個問題實際上首先是關於如何構造一個返回指定結果集的查詢，一旦你有了一個查詢，就可以簡單地將它包裝在一個CREATE VIEW語句中。這裏我會注意到，我們通常不會在MySQL數據庫中使用視圖，這是因爲MySQL優化器的工作方式，它總是將視圖查詢實現爲派生表，這與查看其他視圖查詢的方式不同DBMS）

假設你有單獨的表爲customer，account和/或loan，你可能會需要EXISTS謂語，謂語IN，或結合操作，或某種組合那些。

作爲一個例子，這個查詢（下面）使用EXISTS謂詞來測試在兩個account表和loan表匹配行的存在，並從customer返回行具有至少一個相關聯的帳戶的Perryridge分支，和至少一個相關的貸款。

SELECT c.name 
    FROM customer c 
WHERE EXISTS (SELECT 1 
        FROM account a 
       WHERE a.customer_id = c.id 
        AND a.branch = 'Perryridge' 
      ) 
    AND EXISTS (SELECT 1 
        FROM loan l 
       WHERE l.customer_id = c.id 
      ) 

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這個查詢（下同）返回一個等價的結果集，但使用IN謂詞來尋找帳戶和貸款表的匹配行：

SELECT c.name 
    FROM customer c 
WHERE c.id IN (SELECT a.customer_id 
        FROM account a 
        WHERE a.branch = 'Perryridge' 
       ) 
    AND c.id IN (SELECT l.customer_id 
        FROM loan l 
       ) 

而這個查詢（如下圖）使用JOIN操作來查找匹配的行。 JOIN的行爲與上述查詢不同，因爲它將返回客戶行的多個副本（如果Perryridge分支有多個帳戶或多個貸款）。我們可以通過包含GROUP BY子句輕鬆消除這些重複項。

SELECT c.name 
    FROM customer c 
    JOIN account a 
    ON a.customer_id = c.id 
    AND a.branch = 'Perryridge' 
    JOIN loan l 
    ON l.customer_id = c.id 
GROUP BY c.id 

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缺席有關您的架構的任何信息，我在這裏做了一些假設列id是customer表的主鍵，並且每個account和loan表都有一個外鍵customer_id參考customer(id)等

這些查詢中的每一個都會表現出不同的性能特徵。所有查詢都可能從索引中受益

... ON account (customer_id, branch) 
... ON loan (customer_id) 
... ON customer (id, name) 

這應該足以回答您的問題。

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UPDATE：

鑑於（比利）

branch(branch_name, branch_city, assets) 
customer(customer_name, customer_street, customer_city) 
account(account_number, branch_name, balance) 
loan(loan_number, branch_name, amount) 
depositor(customer_name, account_number) 
borrower(customer_name, loan_number) 

（這似乎WAY更像是一個比它設計和IT專業人士實現的數據庫學術功課或測試問題）

獲得在'Perryr有account的客戶姓名（depositor） IDGE」分支：

SELECT d.customer_name 
    FROM depositor d 
    JOIN account a 
    ON a.account_number = d.account_number 
WHERE a.branch_name = 'Perryridge' 
GROUP BY d.customer_name 

獲取客戶的名稱（borrower）誰在有loan 'Perryridge' 分支：

SELECT b.customer_name 
    FROM borrower b 
    JOIN loan l 
    ON l.loan_number = b.loan_number 
    AND l.branch_name = 'Perryridge' 
GROUP BY b.customer_name 

（如果我們需要從customer表中其他列，我們將增加一個JOIN到該表：

SELECT c.customer_name 
    , c.customer_street 
    , c.customer_city 
    FROM customer c 
    JOIN borrower b 
    ON b.customer_name = c.customer_name 
    JOIN loan l 
    ON l.loan_number = b.loan_number 
    AND l.branch_name = 'Perryridge' 
GROUP 
    BY c.customer_name 
    , c.customer_street 
    , c.customer_city 

獲取誰同時擁有一個存款賬戶（depositor）和貸款客戶的名稱（borrower）在「Perryridge」分支：

SELECT c.customer_name 
    , c.customer_street 
    , c.customer_city 
    FROM customer c 
    JOIN borrower b 
    ON b.customer_name = c.customer_name 
    JOIN loan l 
    ON l.loan_number = b.loan_number 
    AND l.branch_name = 'Perryridge' 
    JOIN depositor d 
    ON d.customer_name = c.customer_name 
    JOIN account a 
    ON a.account_number = d.account_number 
    AND a.branch_name = 'Perryridge' 
GROUP 
    BY c.customer_name 
    , c.customer_street 
    , c.customer_city 
ORDER 
    BY c.customer_name 
    , c.customer_street 
    , c.customer_city 

幾乎相同的結果集可以與使用EXISTS與相關子查詢謂詞的查詢返回。 （不同的是，連接查詢以上可以引進「重複」，當客戶有在Perryridge多個帳戶或一個以上的貸款。GROUP BY子句也消除了重複。）

SELECT c.customer_name 
    , c.customer_street 
    , c.customer_city 
    FROM customer c 
WHERE EXISTS 
     (SELECT 1 
      FROM borrower b 
      WHERE b.customer_name = c.customer_name 
      AND EXISTS 
       (SELECT 1 
        FROM loan l 
        WHERE l.loan_number = b.loan_number 
         AND l.branch_name = 'Perryridge' 
       ) 
     ) 
    AND EXISTS 
     (SELECT 1 
      FROM depositor d 
      WHERE d.customer_name = c.customer_name 
      AND EXISTS 
       (SELECT 1 
        FROM account a 
        WHERE a.account_number = d.account_number 
        AND a.branch_name = 'Perryridge' 
       ) 
     ) 
ORDER 
    BY c.customer_name 
    , c.customer_street 
    , c.customer_city 

這個查詢將不會引入任何重複項，因爲它只返回customer表中的行。 （如果customer表中存在重複行，則此查詢將返回重複項的唯一方法是）。如果customer表中沒有重複行，則GROUP BY不是必需的。