INDEX() 在 MySQL 中创建聚簇索引还是非聚簇索引？
Does INDEX() create a clustered or non-clustered index in MySQL?
我正在学习在 CREATE TABLE 语句中使用 INDEX() 的教程，但没有解释它是集群还是非集群。我的问题是：在 CREATE TABLE 语句中使用 INDEX() 会导致聚簇索引还是非聚簇索引？
例如：
CREATE TABLE test (a varchar(30), b varchar(30), index(a));
/* Is column A a clustered or non-clustered index? */
还想知道如何做相反的事情：如果示例结果是非聚集索引，那么如何编写聚集索引，反之亦然？
TL;DR 主键 - 而 只有 主键 - 是聚集索引。 如果你没有明确定义主键，使用第一个 suitable UNIQUE 键。如果您没有 主键或 suitable UNIQUE 键，MySQL 会生成一个隐藏的聚簇索引。您不能使用 INDEX().
创建聚簇索引
As explained in the docs（强调已添加）：
Every InnoDB table has a special index called the clustered index where the data for the rows is stored. Typically, the clustered index is synonymous with the primary key.
...
When you define a PRIMARY KEY on your table, InnoDB uses it as the clustered index. Define a primary key for each table that you create. If there is no logical unique and non-null column or set of columns, add a new auto-increment column, whose values are filled in automatically.
If you do not define a PRIMARY KEY for your table, MySQL locates the first UNIQUE index where all the key columns are NOT NULL and InnoDB uses it as the clustered index.
If the table has no PRIMARY KEY or suitable UNIQUE index, InnoDB internally generates a hidden clustered index on a synthetic column containing row ID values. The rows are ordered by the ID that InnoDB assigns to the rows in such a table. The row ID is a 6-byte field that increases monotonically as new rows are inserted. Thus, the rows ordered by the row ID are physically in insertion order.
...
All indexes other than the clustered index are known as secondary indexes. In InnoDB, each record in a secondary index contains the primary key columns for the row, as well as the columns specified for the secondary index. InnoDB uses this primary key value to search for the row in the clustered index.
另请参阅 definition of clustered index in the glossary，它将其定义为“主键索引的 InnoDB 术语”，以及一些其他详细信息。
因此，要回答您的问题，除了创建主键或在没有主键的 table 上创建 suitable UNIQUE 键（所有键列都不是 NULL）。 INDEX() 只是创建一个辅助（即非集群）密钥，无论您用它做什么。
* 注意：正如评论中所指出的，其他一些数据库根本没有聚簇索引，而一些数据库允许 table 上有多个聚簇索引。我在回答中只提到 MySQL。
Is column A a clustered or non-clustered index?
非聚集索引，只有主键字段有聚集索引。请记住，table 中只能有一个聚集索引，因此它肯定不能创建一个。
我正在学习在 CREATE TABLE 语句中使用 INDEX() 的教程，但没有解释它是集群还是非集群。我的问题是：在 CREATE TABLE 语句中使用 INDEX() 会导致聚簇索引还是非聚簇索引？
例如：
CREATE TABLE test (a varchar(30), b varchar(30), index(a));
/* Is column A a clustered or non-clustered index? */
还想知道如何做相反的事情：如果示例结果是非聚集索引，那么如何编写聚集索引，反之亦然？
TL;DR 主键 - 而 只有 主键 - 是聚集索引。 如果你没有明确定义主键，使用第一个 suitable UNIQUE 键。如果您没有 主键或 suitable UNIQUE 键，MySQL 会生成一个隐藏的聚簇索引。您不能使用 INDEX().
As explained in the docs（强调已添加）：
Every InnoDB table has a special index called the clustered index where the data for the rows is stored. Typically, the clustered index is synonymous with the primary key.
...
When you define a
PRIMARY KEYon your table, InnoDB uses it as the clustered index. Define a primary key for each table that you create. If there is no logical unique and non-null column or set of columns, add a new auto-increment column, whose values are filled in automatically.If you do not define a
PRIMARY KEYfor your table, MySQL locates the firstUNIQUEindex where all the key columns areNOT NULLand InnoDB uses it as the clustered index.If the table has no
PRIMARY KEYor suitableUNIQUEindex, InnoDB internally generates a hidden clustered index on a synthetic column containing row ID values. The rows are ordered by the ID that InnoDB assigns to the rows in such a table. The row ID is a 6-byte field that increases monotonically as new rows are inserted. Thus, the rows ordered by the row ID are physically in insertion order....
All indexes other than the clustered index are known as secondary indexes. In InnoDB, each record in a secondary index contains the primary key columns for the row, as well as the columns specified for the secondary index. InnoDB uses this primary key value to search for the row in the clustered index.
另请参阅 definition of clustered index in the glossary，它将其定义为“主键索引的 InnoDB 术语”，以及一些其他详细信息。
因此，要回答您的问题，除了创建主键或在没有主键的 table 上创建 suitable UNIQUE 键（所有键列都不是 NULL）。 INDEX() 只是创建一个辅助（即非集群）密钥，无论您用它做什么。
* 注意：正如评论中所指出的，其他一些数据库根本没有聚簇索引，而一些数据库允许 table 上有多个聚簇索引。我在回答中只提到 MySQL。
Is column A a clustered or non-clustered index?
非聚集索引，只有主键字段有聚集索引。请记住，table 中只能有一个聚集索引，因此它肯定不能创建一个。