3.1 子查询 subqueries
关于缩进，可以用www.poorsql.com
inner join
SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDate
FROM Orders
INNER JOIN Customers ON Orders.CustomerID=Customers.CustomerID;多个表格的inner join
Alias：给表格或者某一列暂时命名
给表格命名：
SELECT column_name AS alias_name
FROM table_name;给列命名:
SELECT column_name(s)
FROM table_name AS alias_name;self join自连接
SELECT column_name(s)
FROM table1 T1, table1 T2
WHERE condition;https://www.w3schools.com/sql/sql_join_self.asp 将来自同一个城市的客户分成两列
SELECT A.CustomerName AS CustomerName1, B.CustomerName AS CustomerName2, A.City
FROM Customers A, Customers B
WHERE A.CustomerID <> B.CustomerID
AND A.City = B.City
ORDER BY A.City;
