mysql 更改列大小_如何在MySQL中更改列大小或类型

柔情只为你懂 2022-12-06 15:41 189阅读 0赞

mysql 更改列大小

Just because you made a MySQL column one type or size doesn't mean that it has to stay that way. Changing the column type or size in an existing database is simple.

仅仅因为您将MySQL列设为一种类型或大小并不意味着它必须保持这种状态。 更改现有数据库中的列类型或大小很简单。

## 更改数据库列的大小和类型 **(** Changing a Database Column Size and Type **)** ##

You change a column size or type in MySQL using the [ALTER TABLE][] and MODIFY commands together to make the change.

您可以同时使用[ALTER TABLE][]和MODIFY命令[更改][ALTER TABLE] MySQL的列大小或类型，以进行更改。

Let's say, for example, that you have a column named "State" on a table named "Address" and you previously set it up to hold two characters, expecting people to use 2-character state abbreviations. You find that several people entered entire names instead of 2-character abbreviations, and you want to allow them to do this. You need to make this column larger to allow the full state names to fit. Here is how you do it:

例如，假设您在名为“ Address”的表上有一个名为“ State”的列，并且您之前将其设置为容纳两个字符，并期望人们使用2字符状态缩写。 您发现几个人输入了全名，而不是2个字符的缩写，并且您想允许他们这样做。 您需要使此列变大以允许完整的状态名称适合。 这是您的操作方式：

ALTER TABLE address MODIFY st

In generic terms, you use the ALTER TABLE command followed by the table name, then the MODIFY command followed by the column name and new type and size. Here is an example:

笼统地说，您使用ALTER TABLE命令，后跟表名，然后使用MODIFY命令，后跟列名以及新类型和大小。 这是一个例子：

ALTER TABLE tablename MODIFY columnn

The maximum width of the column is determined by the number in parentheses. The type is identified by VARCHAR as being a variable character field.

列的最大宽度由括号中的数字确定。 该类型由VARCHAR标识为可变字符字段。

## 关于VARCHAR **(** About VARCHAR **)** ##

The VARCHAR(20) in the examples can change to whatever number is appropriate for your column. VARCHAR is a character string of variable length. The maximum length—in this example it is 20—indicates the maximum number of characters you want to store in the column. VARCHAR(25) could store up to 25 characters.

示例中的VARCHAR(20)可以更改为适合您的列的任何数字。 VARCHAR是可变长度的字符串。 最大长度(在此示例中为20)指示要存储在列中的最大字符数。 VARCHAR(25)最多可以存储25个字符。

## ALTER TABLE的其他用途 **(** Other Uses for ALTER TABLE **)** ##

The ALTER TABLE command can also be used to add a new column to a table or to remove an entire column and all its data from a table. For example to add a column, use:

ALTER TABLE命令还可以用于向表中添加新列或从表中删除整个列及其所有数据。 例如，要添加列，请使用：

ALTER

ADD col

To delete a column, use:

要删除列，请使用：

ALTER

DROP C

> 翻译自: [https://www.thoughtco.com/change-columns-size-type-in-mysql-2693875][https_www.thoughtco.com_change-columns-size-type-in-mysql-2693875]

mysql 更改列大小

[ALTER TABLE]: https://www.thoughtco.com/change-column-name-in-mysql-2693874
[https_www.thoughtco.com_change-columns-size-type-in-mysql-2693875]: https://www.thoughtco.com/change-columns-size-type-in-mysql-2693875