70 lines
1.7 KiB
Markdown
70 lines
1.7 KiB
Markdown
---
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title: SQL CREATE INDEX Statement
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---
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The CREATE INDEX statement is used to create indexes in tables.
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Indexes are used to retrieve data from the database very fast. The users cannot see the indexes, they are just used to speed up searches/queries.
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> **Note:** Updating a table with indexes takes more time than updating a table without (because the indexes also need an update). So, only create indexes on columns that will be frequently searched against.
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#### CREATE INDEX Syntax
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Creates an index on a table. Duplicate values are allowed:
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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 Syntax
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Creates a unique index on a table. Duplicate values are not allowed:
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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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> **Note:** The syntax for creating indexes varies among different databases. Therefore: Check the syntax for creating indexes in your database.
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#### CREATE INDEX Example
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The SQL statement below creates an index named "idx_lastname" on the "LastName" column in the "Persons" table:
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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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If you want to create an index on a combination of columns, you can list the column names within the parentheses, separated by commas:
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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 Statement
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The DROP INDEX statement is used to delete an index in a table.
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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:**
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```sql
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DROP INDEX index_name;
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```
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**MySQL:**
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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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