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docs(mysql): clarify UNSIGNED storage requirements #2860

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4 changes: 3 additions & 1 deletion docs/high-performance/sql-optimization.md
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Expand Up @@ -94,13 +94,15 @@ MySQL 提供了两个方法来处理 ip 地址

**b.对于非负型的数据 (如自增 ID,整型 IP，年龄) 来说,要优先使用无符号整型来存储。**

无符号相对于有符号可以多出一倍的存储空间
无符号不会改变整数类型占用的存储字节数，只是把可表示范围从包含负数改为从 0 开始的非负范围；例如 `INT` 无论 signed 还是 unsigned 都占用 4 字节，范围分别如下：

```sql
SIGNED INT -2147483648~2147483647
UNSIGNED INT 0~4294967295
```

具体存储字节数和取值范围可参考 MySQL 官方文档：[Integer Types](https://dev.mysql.com/doc/refman/8.0/en/integer-types.html) 和 [Data Type Storage Requirements](https://dev.mysql.com/doc/refman/8.0/en/storage-requirements.html)。

**c.小数值类型（比如年龄、状态表示如 0/1）优先使用 TINYINT 类型。**

**d.对于日期类型来说， 一定不要用字符串存储日期。可以考虑 DATETIME、TIMESTAMP 和 数值型时间戳。**
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