SQLAlchemy 使用连接
现在我们有两个表，我们将看到如何同时在两个表上创建查询。为了在 Customer 和 Invoice 之间构建一个简单的隐式连接，我们可以使用 Query.filter() 将它们的相关列等同起来。下面，我们使用此方法一次加载 Customer 和 Invoice 实体-
from sqlalchemy.orm import sessionmaker
Session = sessionmaker(bind = engine)
session = Session()
for c, i in session.query(Customer, Invoice).filter(Customer.id == Invoice.custid).all():
print ("ID: {} Name: {} Invoice No: {} Amount: {}".format(c.id,c.name, i.invno, i.amount))
SQLAlchemy 发出的 SQL 表达式如下-
SELECT customers.id
AS customers_id, customers.name
AS customers_name, customers.address
AS customers_address, customers.email
AS customers_email, invoices.id
AS invoices_id, invoices.custid
AS invoices_custid, invoices.invno
AS invoices_invno, invoices.amount
AS invoices_amount
FROM customers, invoices
WHERE customers.id = invoices.custid
以上代码行的结果如下-
ID: 2 Name: Gopal Krishna Invoice No: 10 Amount: 15000
ID: 2 Name: Gopal Krishna Invoice No: 14 Amount: 3850
ID: 3 Name: Govind Pant Invoice No: 3 Amount: 10000
ID: 3 Name: Govind Pant Invoice No: 4 Amount: 5000
ID: 4 Name: Govind Kala Invoice No: 7 Amount: 12000
ID: 4 Name: Govind Kala Invoice No: 8 Amount: 8500
ID: 5 Name: Abdul Rahman Invoice No: 9 Amount: 15000
ID: 5 Name: Abdul Rahman Invoice No: 11 Amount: 6000
使用 Query.join() 方法可以轻松实现实际的 SQL JOIN 语法，如下所示-
session.query(Customer).join(Invoice).filter(Invoice.amount == 8500).all()
join 的 SQL 表达式将显示在控制台上-
SELECT customers.id
AS customers_id, customers.name
AS customers_name, customers.address
AS customers_address, customers.email
AS customers_email
FROM customers JOIN invoices ON customers.id = invoices.custid
WHERE invoices.amount = ?
我们可以使用 for 循环遍历结果-
result = session.query(Customer).join(Invoice).filter(Invoice.amount == 8500)
for row in result:
for inv in row.invoices:
print (row.id, row.name, inv.invno, inv.amount)
以 8500 作为绑定参数，显示以下输出-
Query.join() 知道如何在这些表之间进行连接，因为它们之间只有一个外键。如果没有外键或更多外键，Query.join() 在使用以下形式之一时效果更好-
| query.join(发票, id == Address.custid) |
显式条件 |
| query.join(Customer.invoices) |
从左到右指定关系 |
| query.join(Invoice, Customer.invoices) |
相同，有明确的目标 |
| query.join('发票') |
相同，使用字符串 |
同样可以使用outerjoin()函数来实现左外连接。
query.outerjoin(Customer.invoices)
subquery() 方法生成一个 SQL 表达式，表示嵌入在别名中的 SELECT 语句。
from sqlalchemy.sql import func
stmt = session.query(
Invoice.custid, func.count('*').label('invoice_count')
).group_by(Invoice.custid).subquery()
stmt 对象将包含如下 SQL 语句-
SELECT invoices.custid, count(:count_1) AS invoice_count FROM invoices GROUP BY invoices.custid
一旦我们有了我们的语句，它的行为就像一个表结构。语句中的列可通过名为 c 的属性访问，如下面的代码所示-
for u, count in session.query(Customer, stmt.c.invoice_count).outerjoin(stmt, Customer.id == stmt.c.custid).order_by(Customer.id):
print(u.name, count)
上面的 for 循环显示发票的名称计数如下-
Arjun Pandit None
Gopal Krishna 2
Govind Pant 2
Govind Kala 2
Abdul Rahman 2
