blocks|key|4089437|text|因为其中一个是transact+sql命令(+try+catch)，另一个是DDL语句。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4089438|在执行alter语句之前，最好先查询该列是否存在。|4089439|若要使用MSSQL执行此操作，请参见How+to+check+if+a+column+exists+in+a+SQL+Server+table?|offset|length|4089440|特别针对你的情况，|4089441|IF+COL_LENGTH('nyork',+'Qtr')+IS+NULL
BEGIN
++++alter+table+nyork
++++add+[Qtr]+varchar(20)
END|code-block|syntax|javascript|4089442|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/133031/how-to-check-if-column-exists-in-sql-server-table^0|0|0|I|1I|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|X|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|Y|8|@]|9|@$F|Z|G|10|1|11]]|A|$]]|$1|H|3|I|5|6|7|12|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|13|8|@]|9|@]|A|$M|N]]|$1|O|3|-4|5|6|7|14|8|@]|9|@]|A|$]]]|P|$Q|$5|R|S|T|A|$U|V]]]]

Because one of them is a transact sql command (the try catch) and the other is a DDL statement.
You'd probably do better off querying to see if the column exists before doing the alter statement.
To do this with MSSQL, see <a href="https://stackoverflow.com/questions/133031/how-to-check-if-column-exists-in-sql-server-table">How to check if a column exists in a SQL Server table?</a>
Specifically for your case,
<pre><code>IF COL_LENGTH('nyork', 'Qtr') IS NULL
BEGIN
 alter table nyork
 add [Qtr] varchar(20)
END
</code></pre>

blocks|key|309018|text|你不能这样做。TRY...CATCH只能处理运行时错误。只要列不存在，脚本就会运行，而不是在列已经存在时运行。对象的名称解析是在编译时完成的。因此，SQL+Server将始终在开始执行任何操作之前识别缺少的列。因此，您不能使用动态SQL也做这样的事情。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|309019|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

You cannot do such a thing. TRY...CATCH can only handle runtime errors. Your script will run as long as the column does not exist but not when it is already there. The name resolution of the objects is done at compile time. Therefore SQL Server will always recognize the missing column before it starts any execution. For that reason, you can't also do such a thing with dynamic SQL.

blocks|key|4089516|text|你可以用exec来包装它(‘alter+goes+here')。那么catch就会接住|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4089517|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

You can wrap it with exec('alter goes here'). Then catch will catch

blocks|key|2667805|text|正如@Marcus+Vinicius+Pompeu所说：“在执行alter语句之前，最好先查询该列是否存在。”|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2667806|如果你真的想把TRY...CATCH和DDL一起使用，你可以选择。有两种方法可以做到这一点。|offset|length|style|CODE|BOLD|2667807|基于documentation.的|2667808|在TRY块中使用动态SQL+-已经回答了TRY块中的存储过程|ordered-list-item|2667809|2667810|基于您的代码的示例：|2667811|DROP+PROCEDURE+IF+EXISTS+dbo.sp_my_proc
GO
CREATE+PROCEDURE+dbo.sp_my_proc
AS
++++--Your+original+code+here:
++++ALTER+TABLE+nyork+ADD+[Qtr]+VARCHAR(20)
GO

BEGIN+TRY++
++++EXECUTE+dbo.sp_my_proc+
++++--Optional
++++DROP+PROCEDURE+IF+EXISTS+dbo.sp_my_proc
END+TRY++
BEGIN+CATCH++
++++--Catch+your+error+here
++++SELECT+++
++++++++ERROR_NUMBER()+AS+ErrorNumber++
++++++++,ERROR_MESSAGE()+AS+ErrorMessage;++
END+CATCH;+|code-block|syntax|javascript|2667812|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.microsoft.com/en-us/sql/t-sql/language-elements/try-catch-transact-sql?redirectedfrom=MSDN&view=sql-server-ver15#errors-unaffected-by-a-trycatch-construct^0|0|7|B|J|3|0|2|D|0|0|1|3|K|3|0|0|0|0^^$0|@$1|2|3|4|5|6|7|13|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|14|8|@$D|15|E|16|F|G]|$D|17|E|18|F|H]]|9|@]|A|$]]|$1|I|3|J|5|6|7|19|8|@]|9|@$D|1A|E|1B|1|1C]]|A|$]]|$1|K|3|L|5|M|7|1D|8|@$D|1E|E|1F|F|G]|$D|1G|E|1H|F|G]]|9|@]|A|$]]|$1|N|3|-4|5|6|7|1I|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|1J|8|@]|9|@]|A|$]]|$1|Q|3|R|5|S|7|1K|8|@]|9|@]|A|$T|U]]|$1|V|3|-4|5|6|7|1L|8|@]|9|@]|A|$]]]|W|$X|$5|Y|Z|10|A|$11|12]]]]

As @Marcus Vinicius Pompeu said: &quot;You'd probably do better off querying to see if the column exists before doing the alter statement.&quot;
But, if you really want to use <code>TRY...CATCH</code> with DDL. There is two way for do this.
<ol>
<li>Use dynamic SQL in the <code>TRY</code> block - It has been answered <a href="https://stackoverflow.com/questions/34057380/cannot-catch-sql-server-alter-table-error">here</a>.</li>
<li>Use stored procedure in the <code>TRY</code> block - Based on <a href="https://docs.microsoft.com/en-us/sql/t-sql/language-elements/try-catch-transact-sql?redirectedfrom=MSDN&amp;view=sql-server-ver15#errors-unaffected-by-a-trycatch-construct" rel="nofollow noreferrer">documentation</a>.</li>
</ol>
Example based on your code:
<pre class="lang-sql prettyprint-override"><code>DROP PROCEDURE IF EXISTS dbo.sp_my_proc
GO
CREATE PROCEDURE dbo.sp_my_proc
AS
 --Your original code here:
 ALTER TABLE nyork ADD [Qtr] VARCHAR(20)
GO

BEGIN TRY 
 EXECUTE dbo.sp_my_proc 
 --Optional
 DROP PROCEDURE IF EXISTS dbo.sp_my_proc
END TRY 
BEGIN CATCH 
 --Catch your error here
 SELECT 
 ERROR_NUMBER() AS ErrorNumber 
 ,ERROR_MESSAGE() AS ErrorMessage; 
END CATCH; 
</code></pre>

Why I this can't handle the alter table?

<pre><code>Begin Try
alter table nyork add [Qtr] varchar(20)
End Try
Begin Catch
Print 'Column already exist'
End Catch'
</code></pre>

Try Catch Can't handle alter table

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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为什么我这不能处理alter table？Begin Tryalter table nyork add [Qtr] varchar(20)End TryBegin CatchPrint 'Column already exist'End Catch'

问Try Catch无法处理alter table
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