---
{
    "title": "substring",
    "language": "zh-CN"
}
---

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# substring
## description
### Syntax

`VARCHAR substring(VARCHAR str, INT pos[, INT len])`

没有 `len` 参数时返回从位置 `pos` 开始的字符串 `str` 的一个子字符串，
在有 `len` 参数时返回从位置 `pos` 开始的字符串 `str` 的一个长度为 `len` 子字符串，
`pos` 参数可以使用负值，在这种情况下，子字符串是以字符串 `str` 末尾开始计算 `pos` 个字符，而不是开头,
`pos` 的值为 0 返回一个空字符串。

对于所有形式的 SUBSTRING()，要从中提取子字符串的字符串中第一个字符的位置为1。

## example

```
mysql> select substring('abc1', -2);
+-----------------------------+
| substring('abc1', 2)        |
+-----------------------------+
| bc1                         |
+-----------------------------+

mysql> select substring('abc1', -2);
+-----------------------------+
| substring('abc1', -2)       |
+-----------------------------+
| c1                          |
+-----------------------------+

mysql> select substring('abc1', 5);
+-----------------------------+
| substring('abc1', 5)        |
+-----------------------------+
| NULL                        |
+-----------------------------+

mysql> select substring('abc1def', 2, 2);
+-----------------------------+
| substring('abc1def', 2, 2)  |
+-----------------------------+
| bc                          |
+-----------------------------+
```
## keyword
SUBSTRING