SQL Constraint/Index

最新推荐文章于 2024-08-22 19:16:10 发布

转载最新推荐文章于 2024-08-22 19:16:10 发布 · 143 阅读

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原文链接：http://www.cnblogs.com/xuanyuanchen/p/5934101.html

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#数据库

本文介绍了SQL中用于定义业务规则的约束类型，包括外键、非空、唯一和检查约束，并探讨了如何在列级别或表级别定义这些约束。此外，还详细解释了SQL索引的作用及其创建方式，通过索引可以加快对大型表中数据的检索速度。

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1、SQL Constraint

Integrity Constraints are used to apply business rules for the database tables.

The constraints available in SQL are Foreign Key, Not Null, Unique, Check.

Constraints can be defined in two ways
1) The constraints can be specified immediately after the column definition. This is called column-level definition.
2) The constraints can be specified after all the columns are defined. This is called table-level definition.

2、SQL Index

Index in sql is created on existing tables to retrieve the rows quickly.

When there are thousands of records in a table, retrieving information will take a long time. Therefore indexes are created on columns which are accessed frequently, so that the information can be retrieved quickly. Indexes can be created on a single column or a group of columns. When a index is created, it first sorts the data and then it assigns a ROWID for each row.

Syntax to create Index:

CREATE INDEX index_name
ON table_name (column_name1,column_name2...);

转载于:https://www.cnblogs.com/xuanyuanchen/p/5934101.html