MySQL CREATE TABLE语句中的PRIMARY KEY定义 [英] PRIMARY KEY definition in MySQL CREATE TABLE statement

问题描述

这段代码有什么区别?

CREATE TABLE samples (
  sampleid INT(11) NOT NULL AUTO_INCREMENT,
  sampledate DATE NOT NULL,
  location VARCHAR(25) NOT NULL,
  PRIMARY KEY (sampleid)
)
ENGINE=InnoDB;

和这个:

CREATE TABLE samples (
  sampleid INT(11) NOT NULL AUTO_INCREMENT PRIMARY KEY,
  sampledate DATE NOT NULL,
  location VARCHAR(25) NOT NULL,
)
ENGINE=InnoDB;

代码?

因此，单独的PRIMARY KEY语句或作为列定义的一部分.列定义中对UNIQUE INDEX和UNIQUE关键字有相同的问题.

So a separate PRIMARY KEY statement or as part of a column definition. Same question for UNIQUE INDEX and UNIQUE keyword in column definition.

推荐答案

第二种语法仅仅是一种快捷方式，允许您指定列并在单个子句中为其添加索引.

The second syntax is merely a shortcut allowing you to specify the column and add an index on it in a single clause.

在您只想创建一列并在其上添加索引的情况下，这样做很好.

This works out fine in cases where you simply want to create a column and add an index on it.

如果您想做更复杂的事情，则需要使用第一种语法，例如，基于多个列而不是单个列添加索引，或者要在现有列上添加或更改索引；也就是说，您不会同时创建列和索引.

You'll need to use the first syntax if you want to do something more complicated, such as adding an index based on multiple columns rather than a single column, or if you are adding or changing an index on an existing column; that is, you are not creating the column and the index on it at the same time.

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