MySQL：为什么我需要带“ WHERE ID NOT IN（subquery1 UNION subquery 2）”的额外选择

Question

我有一个与此简化版本相似的表：

CREATE TABLE `accounts` (
  `id` int(11) NOT NULL,
  `account_type_id` int(10) NOT NULL,
  `type` varchar(10) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `accounts` VALUES (1,1,'single'),(2,1,'single'),(3,1,'single'),(4,1,'single'),(5,1,'single'),(6,1,'single'),(7,1,'single'),(8,1,'single'),(9,1,'single'),(10,2,'single'),(11,2,'single'),(12,2,'single'),(13,2,'single'),(14,2,'single'),(15,2,'single'),(16,2,'single'),(17,2,'single'),(18,2,'single'),(19,2,'single'),(20,2,'single'),(21,1,'joint'),(22,1,'joint'),(23,1,'joint'),(24,1,'joint'),(25,1,'joint'),(26,1,'joint'),(27,1,'joint'),(28,1,'joint'),(29,1,'joint'),(30,1,'joint'),(31,2,'joint'),(32,2,'joint'),(33,2,'joint'),(34,2,'joint'),(35,2,'joint'),(36,2,'joint'),(37,2,'joint'),(38,2,'joint'),(39,2,'joint'),(40,2,'joint'),(41,3,'single'),(42,3,'single'),(43,3,'single'),(44,3,'single'),(45,3,'single'),(46,3,'single'),(47,3,'single'),(48,3,'single'),(49,3,'single'),(50,3,'single'),(51,3,'single'),(52,3,'single'),(53,3,'single'),(54,3,'single'),(55,3,'single'),(56,3,'single'),(57,3,'single'),(58,3,'single'),(59,3,'single'),(60,3,'single'),(61,3,'joint'),(62,3,'joint'),(63,3,'joint'),(64,3,'joint'),(65,3,'joint'),(66,3,'joint'),(67,3,'joint'),(68,3,'joint'),(69,3,'joint'),(70,3,'joint'),(71,3,'joint'),(72,3,'joint'),(73,3,'joint'),(74,3,'joint'),(75,3,'joint'),(76,3,'joint'),(77,3,'joint'),(78,3,'joint'),(79,3,'joint'),(80,3,'joint');

我要保留：

• 随机5x类型=单，account_type_id = 1或2
• 随机5x类型=联合，account_type_id = 1或2
• 随机5x类型=单，account_type_id = 3
• 随机5x类型=联合，account_type_id = 3

我的方法是获取与上述每个记录匹配的5条记录的ID，然后删除其他所有记录。

(SELECT id FROM accounts WHERE account_type_id IN (1, 2) AND `type` = 'single' ORDER BY RAND() LIMIT 5)
  UNION
(SELECT id FROM accounts WHERE account_type_id IN (1, 2) AND `type` = 'joint' ORDER BY RAND() LIMIT 5)
  UNION
(SELECT id FROM accounts WHERE account_type_id = 3 AND `type` = 'single' ORDER BY RAND() LIMIT 5)
  UNION
(SELECT id FROM accounts WHERE account_type_id = 3 AND `type` = 'joint' ORDER BY RAND() LIMIT 5)

正确返回每种必需类型的5个ID。 但是，如果我尝试直接在WHERE id NOT IN (...)使用该结果集，则会收到错误消息（对于示例，我已用SELECT替换了DELETE ）：

SELECT * FROM accounts WHERE id NOT IN(
    (SELECT id FROM accounts WHERE account_type_id IN (1, 2) AND `type` = 'single' ORDER BY RAND() LIMIT 5)
      UNION
    (SELECT id FROM accounts WHERE account_type_id IN (1, 2) AND `type` = 'joint' ORDER BY RAND() LIMIT 5)
      UNION
    (SELECT id FROM accounts WHERE account_type_id = 3 AND `type` = 'single' ORDER BY RAND() LIMIT 5)
      UNION
    (SELECT id FROM accounts WHERE account_type_id = 3 AND `type` = 'joint' ORDER BY RAND() LIMIT 5)
);

Error Code: 1064. You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'UNION   (SELECT id FROM accounts WHERE account_type_id IN (1, 2) AND `type` = 'j' at line 3

如果然后我添加一个中间子查询，如下所示：

SELECT * FROM accounts WHERE id NOT IN(
    SELECT a.id FROM (
        (SELECT id FROM accounts WHERE account_type_id IN (1, 2) AND `type` = 'single' ORDER BY RAND() LIMIT 5)
          UNION
        (SELECT id FROM accounts WHERE account_type_id IN (1, 2) AND `type` = 'joint' ORDER BY RAND() LIMIT 5)
          UNION
        (SELECT id FROM accounts WHERE account_type_id = 3 AND `type` = 'single' ORDER BY RAND() LIMIT 5)
          UNION
        (SELECT id FROM accounts WHERE account_type_id = 3 AND `type` = 'joint' ORDER BY RAND() LIMIT 5)
    ) a
);

我得到了想要的结果...请有人解释为什么需要额外的查询吗？

Answer 1

如果您说NOT IN意味着

id不在字段集中（1，2,3,4,5，...）

在您的查询NOT IN中，然后找到并集查询，没有一组值。

但是如果您要进行额外的子查询（将选择a.id），则该子查询已经是一组ID值

那么当您说“不输入”（这些ID）时，它将返回正确的结果。

你明白我的意思。