找出会员等级 tier 缺失的客户。
- 只有
tier为 null 才算缺失；空字符串""和字面上的文字null都不算缺失（第二个 case 里有这样的值）。
输入
customers(customer_id, name, email, city, signup_date, tier)
输出
保留客户表的全部列：customer_id, name, email, city, signup_date, tier，行顺序不限。
找出缺失值
输入表 （case base，共 2 个 case：base、blank）
customers shop/customers
| # | customer_idint64 | namestring | emailstring | citystring | signup_datedate | tierstring |
|---|---|---|---|---|---|---|
| 1 | 1 | Fang Wang | fang.wang@shop.io | Beijing | 2025-03-13 | silver |
| 2 | 2 | Dana Xu | dana.xu@mail.cn | null | 2025-02-17 | silver |
| 3 | 3 | Li Zhou | li.zhou@example.com | Guangzhou | 2025-10-07 | bronze |
| 4 | 4 | Ping Wang | ping.wang@example.com | null | 2025-05-23 | gold |
| 5 | 5 | Tom Zhang | tom.zhang@example.com | Shenzhen | 2025-06-26 | gold |
| 6 | 6 | 王芳 | user06@shop.io | Hangzhou | 2025-02-10 | gold |
| 7 | 7 | Victor Xu | victor.xu@example.com | Shanghai | 2025-12-05 | bronze |
| 8 | 8 | Dana Chen | dana.chen@example.com | Hangzhou | 2025-11-22 | bronze |
| 9 | 9 | Uma Chen | uma.chen@shop.io | Guangzhou | 2025-11-28 | null |
| 10 | 10 | Ivy Zhou | ivy.zhou@shop.io | Hangzhou | 2025-07-14 | gold |
| 11 | 11 | Mia Smith | mia.smith@mail.cn | Shanghai | 2025-04-28 | gold |
| 12 | 12 | Xin Yang | xin.yang@example.com | Chengdu | 2026-01-03 | silver |
| 13 | 13 | Xin Smith | xin.smith@mail.cn | Beijing | 2025-05-07 | silver |
| 14 | 14 | Zoe Xu | zoe.xu@mail.cn | Beijing | 2025-09-18 | silver |
| 15 | 15 | Fang Liu | fang.liu@shop.io | Guangzhou | 2025-08-05 | bronze |
| 16 | 16 | 杨磊 | user16@mail.cn | Shanghai | 2025-12-15 | silver |
| 17 | 17 | Ping Smith | ping.smith@mail.cn | null | 2025-08-11 | bronze |
| 18 | 18 | Oscar Zhou | oscar.zhou@example.com | Shanghai | 2025-11-17 | null |
| 19 | 19 | Kevin Liu | kevin.liu@mail.cn | Chengdu | 2026-02-03 | gold |
| 20 | 20 | Sara Wu | sara.wu@example.com | null | 2025-07-05 | gold |
显示前 20 行，共 50 行
期望输出 （case base，行顺序不限）
| # | customer_idInt64 | nameString | emailString | cityString | signup_dateDate | tierString |
|---|---|---|---|---|---|---|
| 1 | 9 | Uma Chen | uma.chen@shop.io | Guangzhou | 2025-11-28 | null |
| 2 | 18 | Oscar Zhou | oscar.zhou@example.com | Shanghai | 2025-11-17 | null |
| 3 | 21 | Rui Chen | rui.chen@example.com | Chengdu | 2025-02-10 | null |
| 4 | 22 | 刘洋 | user22@example.com | Chengdu | 2026-01-28 | null |
| 5 | 25 | 张伟 | user25@example.com | Shanghai | 2025-06-23 | null |
| 6 | 30 | Eric Chen | eric.chen@example.com | null | 2025-03-13 | null |
| 7 | 31 | Nina Zhang | nina.zhang@mail.cn | Chengdu | 2025-02-20 | null |
| 8 | 40 | Chen Wang | chen.wang@mail.cn | Chengdu | 2025-02-21 | null |
| 9 | 43 | Rui Smith | rui.smith@example.com | Beijing | 2025-05-16 | null |
| 10 | 45 | Oscar Wang | RUI.CHEN@EXAMPLE.COM | Shenzhen | 2025-01-23 | null |
| 11 | 50 | Hao Chen | hao.chen@example.com | Beijing | 2025-07-31 | null |
共 11 行