我有这三张桌子：
“课程表”
id 课程标题 章节数 1 课程1 5 2 课程2 3 “章节表”
id 章节标题 子章节数 course_id --参考课程（id） 1 第 1 章 2 1 2 第2章 4 1 3 第3章 1 1 4 第4章 3 1 5 第5章 0 1 6 第 1 章 4 2 7 第2章 5 2 8 第3章 0 2 “子章节表”
id 子章节标题 chapter_id --参考章节(id) 1 第 1 章 1 2 第2章 1 3 第 1 章 2 4 第2章 2 5 第3章 2 6 第 4 章 2 7 第 1 章 3 ... ........................ ...
我希望结果是这样的 JSON 格式：
{
"course": [
{
"course_id": 1,
"course_title": "course 1",
"chapters_count": 5,
"chapters": [
{
"chapter_id": 1,
"chapter_title": "chapter 1",
"sub_chapters": [
{
"sub_chapter_id": 1,
"sub_chapter_title": "sub chapter 1"
},
{
"sub_chapter_id": 2,
"sub_chapter_title": "sub chapter 2"
},
{
"sub_chapter_id": 3,
"sub_chapter_title": "sub chapter 3"
}
]
},
{
"chapter_id": 2,
"chapter_title": "chapter 2",
"sub_chapters": [
{
"sub_chapter_id": 1,
"sub_chapter_title": "sub chapter 1"
},
{
"sub_chapter_id": 2,
"sub_chapter_title": "sub chapter 2"
}
]
}
]
}
]
}
此类查询最复杂的部分是确定如何构建嵌套聚合，因为 PostgreSQL 不支持嵌套聚合函数。
运行以下 SQL 设置演示环境（注意表不包括下级表关联行的计数）：
CREATE TABLE courses (
id integer GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
course_title text NOT NULL UNIQUE
);
CREATE TABLE chapters (
id integer GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
chapter_title text NOT NULL,
course_id integer NOT NULL REFERENCES courses(id),
UNIQUE (course_id, chapter_title)
);
CREATE TABLE sub_chapters (
id integer GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
sub_chapter_title text NOT NULL,
chapter_id integer NOT NULL REFERENCES chapters(id),
UNIQUE (chapter_id, sub_chapter_title)
);
INSERT INTO courses (id, course_title)
OVERRIDING SYSTEM VALUE
VALUES (1, 'course 1'),
(2, 'course 2');
INSERT INTO chapters (id, chapter_title, course_id)
OVERRIDING SYSTEM VALUE
VALUES (1, 'chapter 1', 1),
(2, 'chapter 2', 1),
(3, 'chapter 3', 1),
(4, 'chapter 4', 1),
(5, 'chapter 5', 1),
(6, 'chapter 1', 2),
(7, 'chapter 2', 2),
(8, 'chapter 3', 2);
INSERT INTO sub_chapters (id, sub_chapter_title, chapter_id)
OVERRIDING SYSTEM VALUE
VALUES (1, 'sub chapter 1', 1),
(2, 'sub chapter 2', 1),
(3, 'sub chapter 1', 2),
(4, 'sub chapter 2', 2),
(5, 'sub chapter 3', 2),
(6, 'sub chapter 4', 2),
(7, 'sub chapter 1', 3);
--- synchronize ID sequences with inserted rows
SELECT
c.table_schema,
c.table_name,
c.column_name,
x.sequence_name,
x.last_value,
x.max_value,
SETVAL(x.sequence_name, GREATEST(x.last_value, x.max_value))
FROM
information_schema.columns c
CROSS JOIN LATERAL XMLTABLE(
'/table/row'
PASSING QUERY_TO_XML(
FORMAT(
$$SELECT %1$L AS sequence_name, last_value, (SELECT MAX(%2$I) FROM %3$I.%4$I) AS max_value FROM %1$s$$,
PG_GET_SERIAL_SEQUENCE(c.table_name, c.column_name),
c.column_name,
c.table_schema,
c.table_name
),
FALSE,
FALSE,
''
)
COLUMNS
sequence_name TEXT,
LAST_VALUE BIGINT,
max_value bigint
) x
WHERE
c.table_schema = CURRENT_SCHEMA()
AND c.table_name IN ('courses', 'chapters', 'sub_chapters')
AND c.column_name = 'id';
以下查询使用
LATERALSELECT
JSON_STRIP_NULLS(
JSON_BUILD_OBJECT(
'course',
JSON_AGG(
JSON_BUILD_OBJECT(
'course_id',
courses.id,
'course_title',
courses.course_title,
'chapters_count',
chaps.chapters_count,
'chapters',
chaps.chapters
)
)
)
) AS courses
FROM
courses
CROSS JOIN LATERAL (
SELECT
COUNT(*) AS chapters_count,
JSON_AGG(
JSON_BUILD_OBJECT(
'chapter_id',
chapters.id,
'chapter_title',
chapters.chapter_title,
'sub_chapters_count',
sc.sub_chapters_count,
'sub_chapters',
sc.sub_chapters
)
) AS chapters
FROM
chapters
CROSS JOIN LATERAL (
SELECT
COUNT(*) AS sub_chapters_count,
JSON_AGG(
JSON_BUILD_OBJECT(
'sub_chapter_id',
sub_chapters.id,
'sub_chapter_title',
sub_chapters.sub_chapter_title
)
ORDER BY
sub_chapters.id
) AS sub_chapters
FROM
sub_chapters
WHERE
sub_chapters.chapter_id = chapters.id
) sc
WHERE
chapters.course_id = courses.id
) chaps;
结果输出为：
{
"course": [
{
"course_id": 1,
"course_title": "course 1",
"chapters_count": 5,
"chapters": [
{
"chapter_id": 1,
"chapter_title": "chapter 1",
"sub_chapters_count": 2,
"sub_chapters": [
{
"sub_chapter_id": 1,
"sub_chapter_title": "sub chapter 1"
},
{
"sub_chapter_id": 2,
"sub_chapter_title": "sub chapter 2"
}
]
},
{
"chapter_id": 2,
"chapter_title": "chapter 2",
"sub_chapters_count": 4,
"sub_chapters": [
{
"sub_chapter_id": 3,
"sub_chapter_title": "sub chapter 1"
},
{
"sub_chapter_id": 4,
"sub_chapter_title": "sub chapter 2"
},
{
"sub_chapter_id": 5,
"sub_chapter_title": "sub chapter 3"
},
{
"sub_chapter_id": 6,
"sub_chapter_title": "sub chapter 4"
}
]
},
{
"chapter_id": 3,
"chapter_title": "chapter 3",
"sub_chapters_count": 1,
"sub_chapters": [
{
"sub_chapter_id": 7,
"sub_chapter_title": "sub chapter 1"
}
]
},
{
"chapter_id": 4,
"chapter_title": "chapter 4",
"sub_chapters_count": 0
},
{
"chapter_id": 5,
"chapter_title": "chapter 5",
"sub_chapters_count": 0
}
]
},
{
"course_id": 2,
"course_title": "course 2",
"chapters_count": 3,
"chapters": [
{
"chapter_id": 6,
"chapter_title": "chapter 1",
"sub_chapters_count": 0
},
{
"chapter_id": 7,
"chapter_title": "chapter 2",
"sub_chapters_count": 0
},
{
"chapter_id": 8,
"chapter_title": "chapter 3",
"sub_chapters_count": 0
}
]
}
]
}
JSON_*JSONB_*