如何修改 MySQL 列以允许 NULL？
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How do I modify a MySQL column to allow NULL?
提问by zmf
MySQL 5.0.45
MySQL 5.0.45
What is the syntax to alter a table to allow a column to be null, alternately what's wrong with this:
更改表以允许列为空的语法是什么，或者这有什么问题：
ALTER mytable MODIFY mycolumn varchar(255) null;
I interpreted the manual as just run the above and it would recreate the column, this time allowing null. The server is telling me I have syntactical errors. I just don't see them.
我将手册解释为只运行上面的内容，它会重新创建列，这次允许为空。服务器告诉我我有语法错误。我只是看不到他们。
回答by Daniel Spiewak
You want the following:
您需要以下内容：
ALTER TABLE mytable MODIFY mycolumn VARCHAR(255);
Columns are nullable by default. As long as the column is not declared UNIQUEor NOT NULL, there shouldn't be any problems.
默认情况下，列可以为空。只要该列未声明UNIQUEor NOT NULL，就不应该有任何问题。
回答by ConroyP
Your syntax error is caused by a missing "table" in the query
您的语法错误是由查询中缺少“表”引起的
ALTER TABLE mytable MODIFY mycolumn varchar(255) null;
回答by Krishnrohit
My solution:
我的解决方案：
ALTER TABLE table_name CHANGE column_name column_name type DEFAULT NULL
For example:
例如：
ALTER TABLE SCHEDULE CHANGE date date DATETIME DEFAULT NULL;
回答by Gerald Senarclens de Grancy
Under some circumstances (if you get "ERROR 1064 (42000): You have an error in your SQL syntax;...") you need to do
在某些情况下（如果你得到“ERROR 1064 (42000): You have an error in your SQL syntax;...”）你需要做
ALTER TABLE mytable MODIFY mytable.mycolumn varchar(255);
回答by Hmerman6006
My solution is the same as @Krishnrohit:
我的解决方案与@Krishnrohit 相同：
ALTER TABLE `table` CHANGE `column_current_name` `new_column_name` DATETIME NULL;
I actually had the column set as NOT NULLbut with the above query it was changed to NULL.
我实际上将列设置为，NOT NULL但是通过上面的查询，它被更改为NULL.
P.S. I know this an old thread but nobody seems to acknowledge that CHANGEis also correct.
PS 我知道这是一个旧线程，但似乎没有人承认这CHANGE也是正确的。
回答by Jan Nejedly
Use:
ALTER TABLE mytable MODIFY mycolumn VARCHAR(255);
用：
ALTER TABLE mytable MODIFY mycolumn VARCHAR(255);
