finish 183

Author: BaffinLee

101-200/183. Customers Who Never Order.md

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+# 183. Customers Who Never Order
+
+- Difficulty: Easy.
+- Related Topics: .
+- Similar Questions: .
+
+## Problem
+
+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       |
++-----------+
+```
+
+## Solution 1
+
+```sql
+# Write your MySQL query statement below
+select a.Name as Customers
+  from Customers a
+  where a.Id not in (
+    select b.CustomerId from Orders b
+  )
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity :
+* Space complexity :
+
+## Solution 2
+
+```sql
+# Write your MySQL query statement below
+select a.Name as Customers
+  from Customers a
+  where not exists (
+    select Id from Orders b where a.Id = b.CustomerId
+  )
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity :
+* Space complexity :
+
+## Solution 3
+
+```sql
+# Write your MySQL query statement below
+select a.Name as Customers
+  from Customers a
+  left join Orders b
+  on a.Id = b.CustomerId
+  where b.CustomerId is null
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity :
+* Space complexity :