73 lines
2.2 KiB
Markdown
73 lines
2.2 KiB
Markdown
---
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{
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"title": "substring",
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"language": "zh-CN"
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}
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---
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<!--
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Licensed to the Apache Software Foundation (ASF) under one
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or more contributor license agreements. See the NOTICE file
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distributed with this work for additional information
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regarding copyright ownership. The ASF licenses this file
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to you under the Apache License, Version 2.0 (the
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"License"); you may not use this file except in compliance
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with the License. You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing,
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software distributed under the License is distributed on an
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"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
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KIND, either express or implied. See the License for the
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specific language governing permissions and limitations
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under the License.
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-->
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# substring
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## description
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### Syntax
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`VARCHAR substring(VARCHAR str, INT pos[, INT len])`
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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，
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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，
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`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,
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`pos` 的值为 0 返回一个空字符串。
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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。
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## example
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```
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mysql> select substring('abc1', -2);
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+-----------------------------+
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| substring('abc1', 2) |
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+-----------------------------+
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| bc1 |
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+-----------------------------+
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mysql> select substring('abc1', -2);
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+-----------------------------+
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| substring('abc1', -2) |
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+-----------------------------+
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| c1 |
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+-----------------------------+
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mysql> select substring('abc1', 5);
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+-----------------------------+
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| substring('abc1', 5) |
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+-----------------------------+
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| NULL |
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+-----------------------------+
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mysql> select substring('abc1def', 2, 2);
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+-----------------------------+
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| substring('abc1def', 2, 2) |
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+-----------------------------+
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| bc |
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+-----------------------------+
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```
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## keyword
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SUBSTRING
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