183. Customers Who Never Order (LeetCode, SQL)

183. Customers Who Never Order

Suppose that a website contains two tables, the Customers table and the Orders table. Write a SQL query to find all customers who never order anything.

Table: Customers.

Id Name 1 Joe 2 Henry 3 Sam 4 Max

Table: Orders.

Id CustomerId 1 3 2 1

Using the above tables as example, return the following:

Customers Henry Max

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解法1：

NOT IN

SELECT Name AS Customers From Customers WHERE Id NOT IN (SELECT CustomerId FROM Orders)

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解法二：

LEFT JOIN

• INNER JOIN（内连接,或等值连接）：获取两个表中字段匹配关系的记录。
• LEFT JOIN（左连接）：获取左表所有记录，即使右表没有对应匹配的记录。
• RIGHT JOIN（右连接）： 与 LEFT JOIN 相反，用于获取右表所有记录，即使左表没有对应匹配的记录。

SELECT Name AS CustomersFROM Customers c  left join Orders o on c.Id=o.CustomerIdWHERE o.CustomerId IS NULL;