What should you pay attention to when using SQL statements in mysql?

1. It is forbidden to use select* and only obtain the necessary fields.

2. Insert must specify a field, and insert into T values() is prohibited.

3. Do not use count (column name) or count (constant) instead of count (*).

4. Foreign keys and cascades are not allowed, and all foreign key concepts must be solved at the application layer.

Example

说明：`NULL` 与任何值的直接比较都为 `NULL`。
1） `NULL<>NULL` 的返回结果是 `NULL`，而不是 `false`。
2） `NULL=NULL` 的返回结果是 `NULL`，而不是 `true`。
3） `NULL<>1` 的返回结果是 `NULL`，而不是 `true`。

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