创建一个视图，其中包含在 Perryridge 分行拥有贷款和帐户的所有客户的姓名。
对此有什么想法吗？我很困惑是使用exists还是in
这些表格是：
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)
【问题讨论】：
标签： mysql
您可以在查询中使用EXISTS 或IN 谓词。要么工作。
(“视图”本质上是一个查询。因此，首先，问题实际上是关于制定一个返回指定结果集的查询。一旦有了查询，只需将其包装在 CREATE VIEW 语句中即可。这里我要注意的是，我们通常不会在 MySQL 数据库中使用视图。这是因为 MySQL 优化器的工作方式，它总是将视图查询具体化为派生表，这与其他 DBMS 中处理视图查询的方式不同.)
假设您有 customer、account 和/或 loan 的单独表，您可能需要 EXISTS 谓词、IN 谓词或连接操作，或它们的某种组合。
作为示例，此查询（如下）使用EXISTS 谓词来测试account 表和loan 表中是否存在匹配行，并从customer 返回至少具有Perryridge 分行的一个关联账户，以及至少一笔关联贷款。
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
)
此查询（如下）返回一个等效的结果集，但使用IN 谓词来查找帐户和贷款表中的匹配行：
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
)
这个查询（如下）利用 JOIN 操作来查找匹配的行。 JOIN 的行为与上面的查询不同，如果在 Perryridge 分行有多个帐户或多个贷款，它将返回客户行的多个副本。我们可以通过包含GROUP BY 子句轻松消除这些重复项。
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
没有关于您的架构的任何信息，我在这里做了一些假设 id 列是 customer 表的主键，并且 account 和 loan 表都有一个外部关键customer_id引用customer(id)等
这些查询中的每一个都会表现出不同的性能特征。所有查询都可能受益于索引
... ON account (customer_id, branch)
... ON loan (customer_id)
... ON customer (id, name)
这应该足以回答您的问题。
更新：
给予（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)
（这似乎更像是一个学术作业或测试问题，而不是由 IT 专业人员设计和实施的数据库。）
获取在“Perryridge”分店拥有account 的客户的姓名 (depositor)：
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
获取在“Perryridge”分店拥有loan 的客户的姓名 (borrower)：
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
（如果我们需要 customer 表中的其他列，我们将向该表添加一个 JOIN：
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
获取在“Perryridge”分行同时拥有存款账户 (depositor) 和贷款 (borrower) 的客户姓名：
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
使用带有相关子查询的 EXISTS 谓词的查询可以返回几乎相同的结果集。 （不同的是，当客户在 Perryridge 拥有多个账户或多个贷款时，上面的 JOIN 查询会引入“重复”。那里的 GROUP BY 子句消除了重复。）
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
此查询不会引入任何重复项，因为它只返回 customer 表中的行。 （此查询返回重复项的唯一方法是customer 表中有重复行。）如果customer 表中没有重复行，则不需要 GROUP BY。
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