从同一个表中选择行数

Question

您好我有一张如下表格，我需要为给定的不同订单ID选择不同商品ID的数量我无法以MySQL格式执行此操作

+-----------+---------+
| OrderID   |TradeID  |
+===========+=========+
| 1         | 58761   |
+-----------+---------+
| 1         | 58762   | 
+-----------+---------+
| 2         | 58763   | 
+-----------+---------+
| 2         | 58764   | 
+-----------+---------+
| 2         | 58765   |
+-----------+---------+

需要结果为

+-----------+---------+
| OrderID   |Count    |
+===========+=========+
| 1         | 2       |
+-----------+---------+
| 2         | 4       | 
+-----------+---------+

Answer 1

只需COUNT / GROUP BY

SELECT OrderID, COUNT(*) 
FROM some_table 
GROUP BY OrderID

SQL小提琴： -

http://www.sqlfiddle.com/#!9/305630/1

请注意，如果您想为每个OrderID使用几个不同的TradeID，则可以使用

SELECT OrderID, COUNT(DISTINCT TradeID) 
FROM some_table 
GROUP BY OrderID