blocks|key|2714181|text|当然有。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2714182|第一个：|2714183|SELECT+null,+null,+null,+null+FROM+items|code-block|syntax|javascript|2714184|我将选择四列，每列中都有NULL值，无论这个表中的值和列是什么，都会选择但是返回的行数将等于items表中的行数，如果items表中有n行，它将返回n行，其中所有列都具有NULL值。|offset|length|style|CODE|2714185|SQL+Fiddle+Demo|2714186|2714187|然而，：|BOLD|2714188|SELECT+*+FROM+items|2714189|将选择具有此表items中的所有值的所有列。|2714190|因此它们是完全不同的。|2714191|在您的示例中，因为只有四列，所以第一个列将始终选择四个列，其中第二个列将使用包含的值选择它们。|2714192|entityMap|0|LINK|mutability|MUTABLE|url|http://www.sqlfiddle.com/#!3/99665/1^0|0|0|0|C|4|1A|5|1N|5|2D|4|0|0|F|0|0|0|3|1|0|0|7|5|0|0|0^^$0|@$1|2|3|4|5|6|7|1A|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|1B|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|1C|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|1D|8|@$K|1E|L|1F|M|N]|$K|1G|L|1H|M|N]|$K|1I|L|1J|M|N]|$K|1K|L|1L|M|N]]|9|@]|A|$]]|$1|O|3|P|5|6|7|1M|8|@]|9|@$K|1N|L|1O|1|1P]]|A|$]]|$1|Q|3|-4|5|6|7|1Q|8|@]|9|@]|A|$]]|$1|R|3|S|5|6|7|1R|8|@$K|1S|L|1T|M|T]]|9|@]|A|$]]|$1|U|3|V|5|F|7|1U|8|@]|9|@]|A|$G|H]]|$1|W|3|X|5|6|7|1V|8|@$K|1W|L|1X|M|N]]|9|@]|A|$]]|$1|Y|3|Z|5|6|7|1Y|8|@]|9|@]|A|$]]|$1|10|3|11|5|6|7|1Z|8|@]|9|@]|A|$]]|$1|12|3|-4|5|6|7|20|8|@]|9|@]|A|$]]]|13|$14|$5|15|16|17|A|$18|19]]]]

OF course there is.

The first one:

<pre><code>SELECT null, null, null, null FROM items
</code></pre>

Will select four columns with <code>NULL</code> values in each, whatever the values and the columns in this table <code>items</code>. But the number of rows returned will equal to the number of rows in the <code>items</code> table, if you have n rows in the <code>items</code> table, it will return n rows with all the columns have <code>NULL</code> values.

<a href="http://www.sqlfiddle.com/#!3/99665/1" rel="nofollow">SQL Fiddle Demo</a>

<hr>

However:

<pre><code>SELECT * FROM items
</code></pre>

Will select all the columns with all the values in this table <code>items</code>.

Se they are totally different. 

In your case since there are only four columns, the first one will always select four <code>NULL</code> columns, where is the second one will select them with the values that contain.

blocks|key|589556|text|是的，每一个都是不同的。当您从表中选择所提供的查询时，表中的列数并不重要。自己看看每个查询的区别。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|589557|589558|从项目中选择null、null、*、null；|ordered-list-item|589559|589560|Click+here+to+see+the+output|offset|length|589561|SELECT+null，null+FROM+items；|unordered-list-item|589562|589563|Click+here+to+see+the+output|589564|SELECT+*+FROM+items；|589565|589566|Click+here+to+see+the+output|589567|entityMap|0|LINK|mutability|MUTABLE|url|http://www.sqlfiddle.com/#!2/593cf/8/0|1|http://www.sqlfiddle.com/#!2/593cf/2/0|2|http://www.sqlfiddle.com/#!2/593cf/1/0^0|0|0|0|0|0|S|0|0|0|0|0|S|1|0|0|0|0|S|2|0^^$0|@$1|2|3|4|5|6|7|17|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|18|8|@]|9|@]|A|$]]|$1|C|3|D|5|E|7|19|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|1A|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|1B|8|@]|9|@$I|1C|J|1D|1|1E]]|A|$]]|$1|K|3|L|5|M|7|1F|8|@]|9|@]|A|$]]|$1|N|3|-4|5|6|7|1G|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|1H|8|@]|9|@$I|1I|J|1J|1|1K]]|A|$]]|$1|Q|3|R|5|M|7|1L|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|1M|8|@]|9|@]|A|$]]|$1|T|3|U|5|6|7|1N|8|@]|9|@$I|1O|J|1P|1|1Q]]|A|$]]|$1|V|3|-4|5|6|7|1R|8|@]|9|@]|A|$]]]|W|$X|$5|Y|Z|10|A|$11|12]]|13|$5|Y|Z|10|A|$11|14]]|15|$5|Y|Z|10|A|$11|16]]]]

Yes, each one is different. The number of columns in a table does not matter when you are selecting from a table for the queries that you have provided. See the difference of each query for yourself.

<ol>
<li>SELECT null, null, *, null FROM items;

<a href="http://www.sqlfiddle.com/#!2/593cf/8/0" rel="nofollow">Click here to see the output</a></li>
<li>SELECT null, null, null, null FROM items;

<a href="http://www.sqlfiddle.com/#!2/593cf/2/0" rel="nofollow">Click here to see the output</a></li>
<li>SELECT * FROM items;

<a href="http://www.sqlfiddle.com/#!2/593cf/1/0" rel="nofollow">Click here to see the output</a></li>
</ol>

blocks|key|6066951|text|第一个查询将显示与表中相同数量的记录。但是在select查询中，这些值将被替换为空值。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|6066952|SELECT+null,+null,+null,+null+FROM+items|code-block|syntax|javascript|6066953|这类似于|6066954|SELECT+5,+5,+5,+5+FROM+items|6066955|第二个查询，|6066956|SELECT+*+FROM+items|6066957|显示带有值的行。|6066958|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|W|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|Y|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

The First query will display the same number of records as in your table. But the values will be replaced by null value in the select query.

<pre><code>SELECT null, null, null, null FROM items
</code></pre>

This is similar to 

<pre><code>SELECT 5, 5, 5, 5 FROM items
</code></pre>

The second query,

<pre><code>SELECT * FROM items
</code></pre>

Diplays row with Values.

blocks|key|2714328|text|除了Mahmoud的回答，我还可以解释一种需要SELECT+NULL的情况。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2714329|假设您有两个表，t1有3列，t2有4列。并且您决定使用UNION编写一个涉及这些表的查询。|2714330|SELECT+*+FROM+++t1
UNION
SELECT+*+FROM+++t2|code-block|syntax|javascript|2714331|上面的查询将不起作用，因为每个表中的列数都不同，因此SQL引擎无法执行联合操作。为了克服这个问题，您可以从t1中选择一个空列，并在访问查询结果集时在您的应用程序代码中正确处理它。|2714332|SELECT+*,+null+FROM+++t1
UNION
SELECT+*+FROM+++t2|2714333|现在，上面的两个查询都将有4列，从而导致一个成功的UNION操作。|2714334|也就是说，如果使用UNION+ALL，相同的查询将工作得很好，因为如果列数不同，它会自动将NULLS添加到结果集中。|2714335|我知道这个例子没有多大意义，但我希望你能理解使用空值的意义。|2714336|entityMap^0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|W|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|X|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|Y|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|Z|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|10|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|11|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|T|$]]

Adding to Mahmoud's answer, I can explain one situation when you would need a SELECT NULL.

Assume you have 2 tables, t1 with 3 columns and t2 with 4 columns. And you decide to write a query with UNION involving these tables.

<pre><code>SELECT * FROM t1
UNION
SELECT * FROM t2
</code></pre>

The above query will not work since number of columns are different in each table and as such, a union operation cannot be performed by the SQL engine. To overcome this, you would select a null column from t1 and handle it correctly in your app code when accessing the query result set.

<pre><code>SELECT *, null FROM t1
UNION
SELECT * FROM t2
</code></pre>

Now both the individual queries above will have 4 columns and thus result in a successful UNION operation.

That said, the same query will work fine if using UNION ALL since it will automatically add NULLS to the result set if the number of columns are different.

I know the example does not make much sense but I hope you understand the point of using nulls.

I'm very new to SQL, and I wasn't able to find a resource describing the difference between * and NULL when selecting from a table. 

For example:

<pre><code>SELECT null, null, *, null FROM items
</code></pre>

Is that any different than:

<pre><code>SELECT null, null, null, null FROM items
</code></pre>

and is this any different than:

<pre><code>SELECT * FROM items
</code></pre>

if there were only four columns to begin with?

What is the difference between * and NULL when selecting from a table in SQL?

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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我对SQL非常陌生，在从表中进行选择时，我找不到描述*和NULL之间区别的资源。例如：SELECT null, null, *, null FROM items这有什么不同于：SELECT null, null, null, null FROM items这有什么不同于：SELECT * FROM items如果一开始只有四列呢？

问在SQL中从表中进行选择时，*和NULL有什么区别？
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问在SQL中从表中进行选择时，*和NULL有什么区别？EN

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问在SQL中从表中进行选择时，*和NULL有什么区别？
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