mysql - mysql根据条件计算子表行

Question

我有两个表：“用户”->“订单”

表：用户

user_id
-----------
    u1
    u2

表：订单

order_id | user_id | flag
-------------------------
    o1   |    u1   |  fA
    o2   |    u2   |  fB

Y 需要获取所有用户，计算有多少次带有标志'fA'的订单

结果我需要什么：

user_id | orders
----------------
   u1   |   1
   u2   |   0

我尝试：

SELECT
    u.user_id,
    COUNT(o.order_id) AS orders
FROM
    `user` AS u LEFT JOIN
    `order` AS o USING (user_id)
WHERE
    o.flag IS NULL OR
    o.flag IN ('fA')
GROUP BY
    u.user_id;

但是，这个查询不包括user=u2因为他没有带有fA标志的订单；我需要user=u2出现orders=0

也许是这样的：

SELECT
    u.user_id,
    COUNT(o.order_id IF o.flag IN('fA')) OR 0 AS count ...

表格和数据：

CREATE TABLE `user` (user_id VARCHAR(2) NULL);
CREATE TABLE `order` (order_id VARCHAR(2) NULL,user_id VARCHAR(2) NULL,flag VARCHAR(2) NULL);
INSERT INTO `user` VALUES ('u1'), ('u2');
INSERT INTO `order` VALUES ('o1','u1','fA'),('o2','u2','fB');

Answer 1

您可以尝试在“LEFT JOIN”的“ON”语句中使用条件，如下所示：

SELECT u.user_id, COUNT(o.user_id)
FROM user u
LEFT JOIN `order` o ON u.user_id = o.user_id AND o.flag = 'fA'
GROUP BY user_id

在 SQLFiddle 中运行示例

Answer 2

至少有三种方法可以做到：

1. 只需使用依赖子查询。

   SELECT u.user_id, (SELECT count(user_id) FROM `order` o WHERE o.user_id = u.user_id AND o.flag = 'fA') count
   FROM `user` u;
   

2. LEFT JOIN在子查询上使用：

   SELECT u.user_id, count(o.user_id) count
   FROM `user` u
   LEFT JOIN (SELECT user_id FROM `order` WHERE flag = 'fA') o USING (user_id)
   GROUP BY u.user_id;
   

3. LEFT JOIN在组合条件下使用：

   SELECT u.user_id, count(o.user_id) count
   FROM `user` u
   LEFT JOIN `order` o ON o.user_id = u.user_id AND o.flag = 'fA'
   GROUP BY u.user_id;
   

我认为最快的是最后一个，但它需要在足够大的数据上进行测试并查看执行计划。

SQL小提琴

Answer 3

您需要使用左外连接而不是左连接

SELECT
    u.user_id,
    COUNT(o.order_id) AS orders
FROM
    `user` AS u LEFT OUTER JOIN
    `order` AS o USING (user_id)
WHERE
    o.flag IS NULL OR
    o.flag IN ('fA')
GROUP BY
    u.user_id;

Answer 4

您可以尝试使用 CASE 语句。我今天没有 MYSQL，所以我可以验证语法，但它应该接近这个：

  SELECT   u.user_id,
             CASE
                WHEN (SELECT COUNT (*)
                        FROM ORDER z
                       WHERE z.user_id = USER.user_id) > 0
                   THEN COUNT (*)
                ELSE 0
             END CASE AS cnt
        FROM USER
    GROUP BY USER.user_id;

Answer 5

此查询将起作用：

SELECT
    u.user_id,
    COUNT(o.order_id) AS orders
FROM
    `user` AS u LEFT OUTER JOIN
    (SELECT user_id, order_id FROM `order` WHERE flag IS NULL OR flag IN ('fA')) as o
    USING (user_id)
GROUP BY
    u.user_id;