74 lines
1.5 KiB
Markdown
74 lines
1.5 KiB
Markdown
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---
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title: SQL CREATE INDEX Statement
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localeTitle: SQL CREATE INDEX语句
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---
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CREATE INDEX语句用于在表中创建索引。
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索引用于非常快速地从数据库中检索数据。用户无法看到索引，它们只是用于加速搜索/查询。
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> **注意：**使用索引更新表比没有表更新表需要更多时间（因为索引也需要更新）。因此，只在经常搜索的列上创建索引。
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#### CREATE INDEX语法
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在表上创建索引。允许重复的值：
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```sql
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CREATE INDEX index_name
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ON table_name (column1, column2, ...);
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```
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#### CREATE UNIQUE INDEX语法
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在表上创建唯一索引。不允许重复值：
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```sql
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CREATE UNIQUE INDEX index_name
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ON table_name (column1, column2, ...);
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```
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> **注意：**创建索引的语法因不同的数据库而异。因此：检查在数据库中创建索引的语法。
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#### CREATE INDEX示例
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下面的SQL语句在“Persons”表的“LastName”列中创建名为“idx\_lastname”的索引：
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```sql
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CREATE INDEX idx_lastname
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ON Persons (LastName);
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```
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如果要在列组合上创建索引，可以在括号中列出列名，用逗号分隔： CREATE INDEX idx\_pname
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```sql
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ON Persons (LastName, FirstName);
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```
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#### DROP INDEX声明
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DROP INDEX语句用于删除表中的索引。
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**MS Access：**
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```sql
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DROP INDEX index_name ON table_name;
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```
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**SQL Server：**
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```sql
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DROP INDEX table_name.index_name;
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```
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**DB2 / Oracle数据库：**
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```sql
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DROP INDEX index_name;
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```
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**MySQL的：**
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```sql
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ALTER TABLE table_name
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DROP INDEX index_name;
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```
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