自己写：把这笔账算完整

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这一节的收官：把反规范化的两面一起量出来。输出四样：现算要过几行、读冗余列要过几行、插一条新选课之后冗余列对不对、重算之后对不对（用 / 隔开）。

enroll_flat（一张宽表，什么都塞在里面）
student  phone  course  teacher  room
阿岚     111    数学    王老师   A101
阿岚     111    英语    李老师   B202
小满     222    数学    王老师   A101
阿泰     333    英语    李老师   B202
小满     222    物理    张老师   C303
南风     444    数学    王老师   A101

拆开之后的三张
student(id, name, phone)          4 行
course (id, name, teacher, room)  3 行
enroll (student_id, course_id)    6 行

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← 自己写：把两种做法的代价摆出来一份 schema 交付前该自查什么 →

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代码编辑器

语言： Ctrl + Enter 运行

import sqlite3
db = sqlite3.connect(":memory:")
db.executescript("""
CREATE TABLE enroll_flat (student TEXT, phone TEXT, course TEXT, teacher TEXT, room TEXT);
CREATE TABLE student (id INTEGER PRIMARY KEY, name TEXT NOT NULL UNIQUE, phone TEXT NOT NULL);
CREATE TABLE course  (id INTEGER PRIMARY KEY, name TEXT NOT NULL UNIQUE,
                      teacher TEXT NOT NULL, room TEXT NOT NULL);
CREATE TABLE enroll  (student_id INTEGER, course_id INTEGER,
                      PRIMARY KEY (student_id, course_id));
""")
db.executemany("INSERT INTO enroll_flat VALUES (?,?,?,?,?)", [
    ("阿岚", "111", "数学", "王老师", "A101"),
    ("阿岚", "111", "英语", "李老师", "B202"),
    ("小满", "222", "数学", "王老师", "A101"),
    ("阿泰", "333", "英语", "李老师", "B202"),
    ("小满", "222", "物理", "张老师", "C303"),
    ("南风", "444", "数学", "王老师", "A101")])
db.executemany("INSERT INTO student VALUES (?,?,?)", [
    (1, "阿岚", "111"), (2, "小满", "222"), (3, "阿泰", "333"), (4, "南风", "444")])
db.executemany("INSERT INTO course VALUES (?,?,?,?)", [
    (1, "数学", "王老师", "A101"), (2, "英语", "李老师", "B202"),
    (3, "物理", "张老师", "C303")])
db.executemany("INSERT INTO enroll VALUES (?,?)", [
    (1, 1), (1, 2), (2, 1), (3, 2), (2, 3), (4, 1)])
db.execute("ALTER TABLE course ADD COLUMN cnt INTEGER")
db.execute("UPDATE course SET cnt = "
           "(SELECT COUNT(*) FROM enroll WHERE course_id = course.id)")

def counted(sql):
    hits = {"n": 0}
    db.create_function("tick", 0, lambda: hits.__setitem__("n", hits["n"] + 1) or 1)
    db.execute(sql).fetchall()
    return hits["n"]

def ok():
    bad = db.execute("SELECT COUNT(*) FROM course c WHERE cnt <> "
                     "(SELECT COUNT(*) FROM enroll WHERE course_id = c.id)"
                     ).fetchone()[0]
    return "对了" if bad == 0 else "不对"

# TODO ①：现算版过几行
a = counted("SELECT course_id, COUNT(*) FROM enroll WHERE  GROUP BY course_id")
# TODO ②：读冗余列版过几行
b = counted("SELECT name, cnt FROM course WHERE ")

db.execute("INSERT INTO enroll VALUES (3, 1)")
c = ok()

# TODO ③：把 cnt 全部重算一遍
db.execute("")
d = ok()

print(str(a) + "/" + str(b) + "/" + c + "/" + d)

本次输入：

输出：

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