主要，唯一和外键约束和索引之间有什么区别？ [英] What is the difference between primary, unique and foreign key constraints, and indexes?

问题描述

主要，唯一和外键约束和索引？

我工作 Oracle 10g 和 SQL Server 2008

推荐答案

主键和唯一键是实体完整性约束

Primary Key and Unique Key are Entity integrity constraints

主要key允许唯一标识表中的每一行，并确保不存在重复行且不输入空值。

Primary key allows each row in a table to be uniquely identified and ensures that no duplicate rows exist and no null values are entered.

唯一键约束用于防止重复表行中的键值，并允许空值。 （在oracle中，一个null不等于另一个null）。

Unique key constraint is used to prevent the duplication of key values within the rows of a table and allow null values. (In oracle one null is not equal to another null).

• KEY或INDEX是指正常的非唯一索引。允许索引的非不同值，因此索引可以包含索引的所有列中具有相同值的行。这些索引不会对您的数据强制执行任何结构，因此它们仅用于加速查询。


• UNIQUE是指索引的所有行必须唯一的索引。也就是说，对于此索引中的所有列，同一行可能不具有与另一行相同的非NULL值。除了用于加速查询之外，UNIQUE索引还可用于强制执行数据结构，因为数据库系统在插入或更新数据时不允许破坏此不同值规则。您的数据库系统可能允许对允许NULL值的列使用UNIQUE索引，在这种情况下，如果两行都包含NULL值（NULL被认为不等于自身），则允许两行相同，尽管这可能是不合需要的，具体取决于您的应用程序。


• PRIMARY的行为与UNIQUE索引完全相同，只是它总是命名为'PRIMARY'，并且表中可能只有一个（并且应该总是有一个;尽管有些数据库系统不强制执行此操作）。 PRIMARY索引旨在唯一标识表中的任何行，因此不应在允许NULL值的任何列上使用它。您的PRIMARY索引应始终位于足以唯一标识行的最小列数上。通常，这只是一个包含唯一自动递增数字的列，但如果还有其他任何可以唯一标识行的列，例如国家/地区列表中的countrycode，则可以使用该列。


• FULLTEXT索引与上述所有索引都不同，它们在数据库系统之间的行为也有所不同。与上述三个（通常是b-tree（允许从最左列开始选择，排序或范围）或散列（允许从最左列开始选择）不同，FULLTEXT索引仅对使用MATCH（）/ AGAINST（）子句。

• KEY or INDEX refers to a normal non-unique index. Non-distinct values for the index are allowed, so the index may contain rows with identical values in all columns of the index. These indexes don't enforce any structure on your data so they are used only for speeding up queries.
• UNIQUE refers to an index where all rows of the index must be unique. That is, the same row may not have identical non-NULL values for all columns in this index as another row. As well as being used to speed up queries, UNIQUE indexes can be used to enforce structure on data, because the database system does not allow this distinct values rule to be broken when inserting or updating data. Your database system may allow a UNIQUE index on columns which allow NULL values, in which case two rows are allowed to be identical if they both contain a NULL value (NULL is considered not equal to itself), though this is probably undesirable depending on your application.
• PRIMARY acts exactly like a UNIQUE index, except that it is always named 'PRIMARY', and there may be only one on a table (and there should always be one; though some database systems don't enforce this). A PRIMARY index is intended as a way to uniquely identify any row in the table, so it shouldn't be used on any columns which allow NULL values. Your PRIMARY index should always be on the smallest number of columns that are sufficient to uniquely identify a row. Often, this is just one column containing a unique auto-incremented number, but if there is anything else that can uniquely identify a row, such as "countrycode" in a list of countries, you can use that instead.
• FULLTEXT indexes are different to all of the above, and their behaviour differs more between database systems. Unlike the above three, which are typically b-tree (allowing for selecting, sorting or ranges starting from left most column) or hash (allowing for selection starting from left most column), FULLTEXT indexes are only useful for full text searches done with the MATCH() / AGAINST() clause.

参见 MySQL中的INDEX，PRIMARY，UNIQUE，FULLTEXT之间的差异？

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