mysql

mysql中为什么删除一个表中的主键约束后该字段不能为空?

SQL语句执行顺序

1. From
2. ON
3. JOIN
4. WHERE
5. GROUP BY
6. SELECT
7. HAVING
8. ORDER BY
9. LIMIT

原文：https://www.cnblogs.com/haizinihao/p/14753095.html