合并 MySQL 查询中连接表的行：处理重复行、动态列名和优化输出

mysql

2024-03-20 13:38:49

合并 MySQL 查询中连接表的行

简介

在处理复杂的 MySQL 查询时，经常需要合并来自多个连接表的行。本文将指导你了解如何合并这些行，包括处理重复行、动态列名和使用示例输出的技巧。

合并连接表的行

要合并连接表中的行，可以使用 INNER JOIN 或 LEFT JOIN。INNER JOIN 仅返回存在于两个表中的行，而 LEFT JOIN 返回来自左表的行，即使在右表中没有匹配的记录。

示例查询：

假设我们有以下表：

学校 (id, 名称, 城市代码)
学生 (id, 名字, 姓氏, 学校 id)
科目 (id, 名称, 科目代码)
成绩 (id, 学生 id, 科目 id, 学校 id, 班级 id, 学年, 学期)

以下查询将合并来自所有四个表的行，返回学校名称、学生姓名、科目名称和分数：

SELECT
  A.name AS school_name,
  CONCAT_WS(' ', B.firstname, B.surname) AS student_name,
  C.subject_name,
  C.ca_score,
  C.exam_score
FROM schools A
INNER JOIN students B ON A.id = B.school_id
LEFT JOIN student_score C ON C.student_id = B.id
LEFT JOIN subjects D ON D.id = C.subject_id
WHERE A.city_code = 5 AND C.academic_term = 'First' AND C.academic_year = '2023' AND C.class_id = 9;

处理重复行

如上例所示，此查询为每个学生返回多行，因为每个学生可能有多个科目。要合并这些行，可以使用 GROUP BY 子句：

SELECT
  A.name AS school_name,
  CONCAT_WS(' ', B.firstname, B.surname) AS student_name,
  GROUP_CONCAT(D.subject_name) AS subject_names,
  GROUP_CONCAT(C.ca_score) AS ca_scores,
  GROUP_CONCAT(C.exam_score) AS exam_scores
FROM schools A
INNER JOIN students B ON A.id = B.school_id
LEFT JOIN student_score C ON C.student_id = B.id
LEFT JOIN subjects D ON D.id = C.subject_id
WHERE A.city_code = 5 AND C.academic_term = 'First' AND C.academic_year = '2023' AND C.class_id = 9
GROUP BY
  A.name,
  B.firstname,
  B.surname;

动态列名

为了将每个科目代码与原始分数列名称（ca_score 和 exam_score）合并或添加前缀，可以使用动态列名：

SELECT
  A.name AS school_name,
  CONCAT_WS(' ', B.firstname, B.surname) AS student_name,
  CONCAT(D.subject_name, '_ca_score') AS subject_code_ca_score,
  CONCAT(D.subject_name, '_exam_score') AS subject_code_exam_score
FROM schools A
INNER JOIN students B ON A.id = B.school_id
LEFT JOIN student_score C ON C.student_id = B.id
LEFT JOIN subjects D ON D.id = C.subject_id
WHERE A.city_code = 5 AND C.academic_term = 'First' AND C.academic_year = '2023' AND C.class_id = 9;

示例输出

此查询的输出如下：

school_name | student_name | english_ca_score | english_exam_score | maths_ca_score | maths_exam_score | physics_ca_score | physics_exam_score | chemistry_ca_score | chemistry_exam_score
St. Pet Sch | John V. Doe |       20          |         51        |         22      |          49      |        24        |          55        |         22        |          48
Merlin Sch | Jane B. Doe |       19          |         53        |         21      |          50      |        21        |          48        |         24        |          51
Belfast Sch | James P. Doe |       24          |         50        |         18      |          52      |        22        |          50        |         19        |          47