如何从以列为中心 table 和以行为中心 table 构建查询？

Question

我想构建一个结合来自两个 table 的数据的查询。

test_1 table:

+----+---------+
| id | field_A |
+----+---------+
|  1 |       1 |
|  2 |       1 |
|  3 |       2 |
|  4 |       2 |
+----+---------+

test_2 table:

+----+---------+---------+---------+
| id | field_A | field_B | field_C |
+----+---------+---------+---------+
|  1 |       1 |       1 | baz     |
|  2 |       1 |       2 | zoo     |
|  3 |       2 |       1 | bin     |
|  4 |       2 |       2 | won     |
+----+---------+---------+---------+

field_A 是两个 table 之间的公共字段，我有一个桥（？） table (test_3) 位于中间：

+----+------+
| id | desc |
+----+------+
|  1 | foo  |
|  2 | bar  |
+----+------+

查询结果如下：

+----+---------+------------------+------------------+
| id | field_A | test_2_field_B_1 | test_2_field_B_2 |
+----+---------+------------------+------------------+
|  1 |       1 | baz              | zoo              |
|  2 |       1 | baz              | zoo              |
|  3 |       2 | bin              | won              |
|  4 |       2 | bin              | won              |
+----+---------+------------------+------------------+

其中：

id 和 field_A 复制自 test_1
test_2_field_B_1 是 test_1.field_A = test_2.field_A 和 test_2.field_B = 1
test_2_field_B_2 是 test_1.field_A = test_2.field_A 和 test_2.field_B = 2

本质上，我需要对以行为中心的 test_2 中的数据进行逆透视，然后将其与以列为中心的 test_1.

合并

我已经尝试按照 post here 进行操作，结果达到了：

SELECT
    t1.id_,
    t1.field_A,
    (SELECT field_C FROM test_2 WHERE field_A = 1 AND field_A = t1.field_A) AS test_2_field_B_1,
    (SELECT field_C FROM test_2 WHERE field_A = 2 AND field_A = t1.field_A) AS test_2_field_B_2
FROM test_1 AS t1
GROUP BY t1.id_

但是，我得到的只是 Workbench 上 Output 框中的 OK。

我真的很想得到一个基于查询的解决方案（而不是视图），因为最终我的目标是使用 SQLAlchemy 将其转换为 Python。

这是创建三个 table 的 SQL：

/*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */;
/*!40101 SET @OLD_CHARACTER_SET_RESULTS=@@CHARACTER_SET_RESULTS */;
/*!40101 SET @OLD_COLLATION_CONNECTION=@@COLLATION_CONNECTION */;
/*!50503 SET NAMES utf8 */;
/*!40103 SET @OLD_TIME_ZONE=@@TIME_ZONE */;
/*!40103 SET TIME_ZONE='+00:00' */;
/*!40014 SET @OLD_UNIQUE_CHECKS=@@UNIQUE_CHECKS, UNIQUE_CHECKS=0 */;
/*!40014 SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0 */;
/*!40101 SET @OLD_SQL_MODE=@@SQL_MODE, SQL_MODE='NO_AUTO_VALUE_ON_ZERO' */;
/*!40111 SET @OLD_SQL_NOTES=@@SQL_NOTES, SQL_NOTES=0 */;

--
-- Table structure for table `test_1`
--

DROP TABLE IF EXISTS `test_1`;
/*!40101 SET @saved_cs_client     = @@character_set_client */;
/*!50503 SET character_set_client = utf8mb4 */;
CREATE TABLE `test_1` (
  `id_` int NOT NULL,
  `field_A` int DEFAULT NULL,
  PRIMARY KEY (`id_`),
  KEY `fk1_idx` (`field_A`),
  CONSTRAINT `fk1` FOREIGN KEY (`field_A`) REFERENCES `test_3` (`id_`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;
/*!40101 SET character_set_client = @saved_cs_client */;

--
-- Dumping data for table `test_1`
--

LOCK TABLES `test_1` WRITE;
/*!40000 ALTER TABLE `test_1` DISABLE KEYS */;
INSERT INTO `test_1` VALUES (1,1),(2,1),(3,2),(4,2);
/*!40000 ALTER TABLE `test_1` ENABLE KEYS */;
UNLOCK TABLES;

--
-- Table structure for table `test_2`
--

DROP TABLE IF EXISTS `test_2`;
/*!40101 SET @saved_cs_client     = @@character_set_client */;
/*!50503 SET character_set_client = utf8mb4 */;
CREATE TABLE `test_2` (
  `id_` int NOT NULL,
  `field_A` int DEFAULT NULL,
  `field_B` int DEFAULT NULL,
  `field_C` varchar(3) DEFAULT NULL,
  PRIMARY KEY (`id_`),
  KEY `fk2_idx` (`field_A`),
  CONSTRAINT `fk2` FOREIGN KEY (`field_A`) REFERENCES `test_3` (`id_`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;
/*!40101 SET character_set_client = @saved_cs_client */;

--
-- Dumping data for table `test_2`
--

LOCK TABLES `test_2` WRITE;
/*!40000 ALTER TABLE `test_2` DISABLE KEYS */;
INSERT INTO `test_2` VALUES (1,1,1,'baz'),(2,1,2,'zoo'),(3,2,1,'bin'),(4,2,2,'won');
/*!40000 ALTER TABLE `test_2` ENABLE KEYS */;
UNLOCK TABLES;

--
-- Table structure for table `test_3`
--

DROP TABLE IF EXISTS `test_3`;
/*!40101 SET @saved_cs_client     = @@character_set_client */;
/*!50503 SET character_set_client = utf8mb4 */;
CREATE TABLE `test_3` (
  `id_` int NOT NULL,
  `desc` varchar(45) DEFAULT NULL,
  PRIMARY KEY (`id_`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;
/*!40101 SET character_set_client = @saved_cs_client */;

--
-- Dumping data for table `test_3`
--

LOCK TABLES `test_3` WRITE;
/*!40000 ALTER TABLE `test_3` DISABLE KEYS */;
INSERT INTO `test_3` VALUES (1,'foo'),(2,'bar');
/*!40000 ALTER TABLE `test_3` ENABLE KEYS */;
UNLOCK TABLES;
/*!40103 SET TIME_ZONE=@OLD_TIME_ZONE */;

/*!40101 SET SQL_MODE=@OLD_SQL_MODE */;
/*!40014 SET FOREIGN_KEY_CHECKS=@OLD_FOREIGN_KEY_CHECKS */;
/*!40014 SET UNIQUE_CHECKS=@OLD_UNIQUE_CHECKS */;
/*!40101 SET CHARACTER_SET_CLIENT=@OLD_CHARACTER_SET_CLIENT */;
/*!40101 SET CHARACTER_SET_RESULTS=@OLD_CHARACTER_SET_RESULTS */;
/*!40101 SET COLLATION_CONNECTION=@OLD_COLLATION_CONNECTION */;
/*!40111 SET SQL_NOTES=@OLD_SQL_NOTES */;

Answer 1

我想你想要：

select t1.id, t1.field_a,
    (select t2.field_c from test_2 t2 where t2.field_a = t1.field_a and t2.field_b = 1) test_2_field_b_1,
    (select t2.field_c from test_2 t2 where t2.field_a = t1.field_a and t2.field_b = 2) test_2_field_b_2
from test_1 t1

您可以使用单个连接和条件聚合实现相同的逻辑：

select t1.id, t1.field_a,
    max(case when t2.field_b = 1 then t2.field_c end) test_2_field_b_1,
    max(case when t2.field_b = 2 then t2.field_c end) test_2_field_b_2
from test_1 t1
inner join test_2 t2 on t2.field_a = t1.field_a
group by t1.id, t1.field_a

如何从以列为中心 table 和以行为中心 table 构建查询？

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