mysql - 子查询选择字段

Question

我试图在 MySQL selct 语句中进行子选择，例如如下所示：

SELECT
  c.id AS id,
  c.name AS name,
  (SELECT count(*) FROM orders o WHERE o.user_id = c.id) AS order_count
FROM
  customers c

任何想法为什么这在 MySQL 中不起作用？是否可以在 MySQL 中做类似的事情？在版本 3.23.58 和 5.1.60 中尝试过。

提前致谢！

Answer 1

你错过了,之后name，试试这个：

SELECT
  c.id AS id,
  c.name AS name,
  (SELECT count(*) FROM orders o WHERE o.user_id = c.id) AS order_count
FROM
  customers c

Answer 2

为了避免这些类型的错误（比如缺少逗号），我喜欢这样写查询......

    SELECT c.id 
         , c.name 
         , COALESCE(COUNT(o.user_id),0) order_count
      FROM customers c
      LEFT
      JOIN orders o
        ON o.user_id = c.id
     GROUP
        BY c.id;

Answer 3

SELECT
  c.id AS id ,
  c.name AS name ,
 (SELECT count(*) 
   FROM orders o 
   WHERE o.user_id = c.id) AS order_count
FROM
 customers c

在每列之后使用“，”

Answer 4

另一种方法使用joins

• SQLFIDDLE 演示

样本数据：

ID  NAME
1   john
2   jack
3   kelly

ID  USERID
10  1
11  2
12  1
13  3
14  2
15  3
16  2
17  4
18  1

询问：

SELECT count(o.userid), c.id, c.name
FROM cusomter c
LEFT JOIN orders o
ON c.id = o.userid
GROUP BY c.id
;

case when如果客户没有任何订单，您可以添加将 null 表示为 0。你的选择。

结果：

COUNT(O.USERID)     ID  NAME
3                   1   john
3                   2   jack
2                   3   kelly