blocks|key|147386|text|这在SQL-92标准中是不允许的，但在MySQL中是允许的。来自MySQL手册|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|147387|MySQL允许您创建类型为CHAR(0)的列。这主要是当您必须与依赖于列的存在而不实际使用其值的旧应用程序兼容时，这是非常有用的。当您需要一个只能接受两个值的列时，CHAR(0)也是非常好的:定义为CHAR(0)+NULL的列只占用一位，只能使用NULL和''+(空字符串)值。|blockquote|style|CODE|147388|entityMap|0|LINK|mutability|MUTABLE|url|http://www.contrib.andrew.cmu.edu/~shadow/sql/sql1992.txt|1|http://dev.mysql.com/doc/refman/5.0/en/string-type-overview.html^0|2|6|0|W|7|1|0|D|7|2A|7|2R|7|3F|4|3K|2|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@$A|T|B|U|1|V]|$A|W|B|X|1|Y]]|C|$]]|$1|D|3|E|5|F|7|Z|8|@$A|10|B|11|G|H]|$A|12|B|13|G|H]|$A|14|B|15|G|H]|$A|16|B|17|G|H]|$A|18|B|19|G|H]]|9|@]|C|$]]|$1|I|3|-4|5|6|7|1A|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]|Q|$5|L|M|N|C|$O|R]]]]

It's not allowed per the <a href="http://www.contrib.andrew.cmu.edu/~shadow/sql/sql1992.txt" rel="noreferrer">SQL-92</a> standard, but permitted in MySQL. From the <a href="http://dev.mysql.com/doc/refman/5.0/en/string-type-overview.html" rel="noreferrer">MySQL manual</a>:

<blockquote>
 MySQL permits you to create a column of type <code>CHAR(0)</code>. This is useful primarily when you have to be compliant with old applications that depend on the existence of a column but that do not actually use its value. <code>CHAR(0)</code> is also quite nice when you need a column that can take only two values: A column that is defined as <code>CHAR(0)</code> NULL occupies only one bit and can take only the values <code>NULL</code> and <code>''</code> (the empty string).
</blockquote>

blocks|key|869949|text|只要检查了MySQL，它确实允许零长度CHAR和VARCHAR.|type|unstyled|depth|inlineStyleRanges|entityRanges|data|869950|这并不是说它非常有用，但我可以想到一种情况，当您不再需要时，将一列截断为0长度，但是您不想破坏在那里写东西的现有代码。您分配给0长度列的任何内容都将被截断并发出警告，但是警告不是错误，它们不会破坏任何东西。|869951|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

Just checked MySQL, it's true that it allows zero-length CHAR and VARCHAR. 

Not that it can be extremely useful but I can think of a situation when you truncate a column to 0 length when you no longer need it but you don't want to break existing code that writes something there. Anything you assign to a 0-length column will be truncated and a warning issued, but warnings are not errors, they don't break anything.

blocks|key|147430|text|由于它们是类似的类型，char和varchar，我将冒昧地猜测varchar(0)的用例与char(0)相同。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|147431|来自字符串类型的文档|147432|MySQL允许您创建CHAR(0)类型的列。这主要是当您必须与依赖于列的存在而不实际使用其值的旧应用程序兼容时，这是非常有用的。当您需要一个只能接受两个值的列时，CHAR(0)也是非常好的:定义为CHAR(0)+NULL的列只占用一个位，并且只能接受NULL和'‘值(空字符串)。|blockquote|147433|entityMap|0|LINK|mutability|MUTABLE|url|http://dev.mysql.com/doc/refman/5.0/en/string-type-overview.html^0|B|4|G|7|V|A|19|7|0|2|5|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|11|8|@]|D|@$9|12|A|13|1|14]]|E|$]]|$1|H|3|I|5|J|7|15|8|@]|D|@]|E|$]]|$1|K|3|-4|5|6|7|16|8|@]|D|@]|E|$]]]|L|$M|$5|N|O|P|E|$Q|R]]]]

As they're similar types, <code>char</code> and <code>varchar</code>, I'm going to venture to guess that the use-case of <code>varchar(0)</code> is the same as <code>char(0)</code>.

From the documentation of <a href="http://dev.mysql.com/doc/refman/5.0/en/string-type-overview.html" rel="nofollow">String Types</a>:

<blockquote>
 MySQL permits you to create a column of type CHAR(0). This is useful
 primarily when you have to be compliant with old applications that
 depend on the existence of a column but that do not actually use its
 value. CHAR(0) is also quite nice when you need a column that can take
 only two values: A column that is defined as CHAR(0) NULL occupies
 only one bit and can take only the values NULL and '' (the empty
 string).
</blockquote>

blocks|key|1148633|text|如果您想标记表中的一个特定行(例如，因为它是默认的)，它与唯一的索引组合使用。唯一索引确保所有其他行都必须是null，因此始终可以通过该列检索行。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1148634|entityMap^0|1I|4|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

It's useful in combination with a unique index for if you want to mark one specific row in your table (for instance, because it serves as a default). The unique index ensures that all other rows have to be <code>null</code>, so you can always retrieve the row by that column.

blocks|key|869964|text|您可以使用它来存储布尔值。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|869965|看下面的代码：|869966|mysql>+create+table+chartest(a+char(0));
Query+OK,+0+rows+affected+(0.26+sec)

mysql>+insert+into+chartest+value(NULL);
Query+OK,+1+row+affected+(0.01+sec)

mysql>+insert+into+chartest+value('');
Query+OK,+1+row+affected+(0.00+sec)

mysql>+select+'true'+from+chartest+where+a+is+null;
%2B------%2B
%7C+true+%7C
%2B------%2B
%7C+true+%7C
%2B------%2B
1+row+in+set+(0.00+sec)

mysql>+select+'false'+from+chartest+where+a+is+not+null;
%2B-------%2B
%7C+false+%7C
%2B-------%2B
%7C+false+%7C
%2B-------%2B
1+row+in+set+(0.00+sec)|code-block|syntax|javascript|869967|我们可以使用NULL来表示真，用'‘(空字符串)表示false！|offset|length|style|CODE|869968|根据MySQL参考手册，只有NULL占用一位。|869969|entityMap^0|0|0|0|6|4|Q|5|0|E|4|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|V|8|@$K|W|L|X|M|N]|$K|Y|L|Z|M|N]]|9|@]|A|$]]|$1|O|3|P|5|6|7|10|8|@$K|11|L|12|M|N]]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|13|8|@]|9|@]|A|$]]]|R|$]]

You can use it to store boolean values. 

Look this code: 

<pre><code>mysql&gt; create table chartest(a char(0));
Query OK, 0 rows affected (0.26 sec)

mysql&gt; insert into chartest value(NULL);
Query OK, 1 row affected (0.01 sec)

mysql&gt; insert into chartest value('');
Query OK, 1 row affected (0.00 sec)

mysql&gt; select 'true' from chartest where a is null;
+------+
| true |
+------+
| true |
+------+
1 row in set (0.00 sec)

mysql&gt; select 'false' from chartest where a is not null;
+-------+
| false |
+-------+
| false |
+-------+
1 row in set (0.00 sec)
</code></pre>

We can use <code>NULL</code> to represent true and '' (empty string) to represent <code>false</code>! 

According to MySQL reference manual, only <code>NULL</code> occupies one bit.

I recently encountered a problem caused by a typo in the database creation script, whereby a column in the database was created as <code>varchar(0)</code> instead of <code>varchar(20)</code>.

I expected that I would have gotten an error for 0-length string field, but I didn't. What is the purpose of <code>varchar(0)</code> or <code>char(0)</code> as I wouldn't be able to store any data in this column anyway.

What's the purpose of varchar(0)

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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最近，我遇到了一个由数据库创建脚本中的一个错误引起的问题，根据这个错误，数据库中的一个列被创建为varchar(0)而不是varchar(20)。我以为我会得到一个0长度字符串字段的错误，但我没有。varchar(0)或char(0)的目的是什么，因为我无论如何都无法在本专栏中存储任何数据。

问varchar(0)的目的是什么？
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