空值：IS NULL / IS NOT NULL

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57.12 空值：IS NULL / IS NOT NULL

NULL 表示「未知/没有值」，与空字符串 '' 不同。

判断 NULL 必须用 IS NULL 或 IS NOT NULL，WHERE col = NULL 永远不对。

查缺邮箱、有备注

import pymysql

conn = pymysql.connect(host='127.0.0.1', user='root', password='root', database='py_demo', charset='utf8mb4')
try:
    with conn.cursor() as cur:
        cur.execute('SELECT name, email FROM students WHERE email IS NULL')
        print('无邮箱：', cur.fetchall())
        cur.execute('SELECT name, remark FROM students WHERE remark IS NOT NULL')
        print('有备注：', cur.fetchall())
finally:
    conn.close()