LeetCode-MySQL-584. 尋找用戶推薦人

給定表 customer ，裏面保存了全部客戶信息和他們的推薦人。code +------+------+-----------+ | id | name | referee_id| +------+------+-----------+ | 1 | Will | NULL | | 2 | Jane | NULL | | 3 | Alex |

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