修改表以修改列的默认值

I have a requirement where we need to modify a column's default value in database table. The table is already an existing table in database and currently the default value of the column is NULL. Now if add a new default value to this column, If I am correct it updates all the existing NULLs of the column to new DEfault value. Is there a way to not to do this but still set a new default value on column. I mean I do not want the existing NULLs to be updated and want them to remain as NULLs.

我需要修改数据库表中列的默认值。该表已经是数据库中的现有表，当前列的默认值为NULL。现在，如果向该列添加一个新的默认值，如果我是正确的，那么它会将该列的所有现有空值更新为新的默认值。是否有办法不这样做，但仍然在列上设置新的默认值。我的意思是，我不希望更新现有的null，希望它们保持为NULLs。

Any help on this is appreciated. Thanks

如有任何帮助，我们将不胜感激。谢谢

4 个解决方案

#1

Your belief about what will happen is not correct. Setting a default value for a column will not affect the existing data in the table.

你对将要发生的事的看法是不正确的。为列设置默认值不会影响表中的现有数据。

I create a table with a column col2 that has no default value

我用一个没有默认值的列col2创建一个表。

SQL> create table foo(
  2    col1 number primary key,
  3    col2 varchar2(10)
  4  );

Table created.

SQL> insert into foo( col1 ) values (1);

1 row created.

SQL> insert into foo( col1 ) values (2);

1 row created.

SQL> insert into foo( col1 ) values (3);

1 row created.

SQL> select * from foo;

      COL1 COL2
---------- ----------
         1
         2
         3

If I then alter the table to set a default value, nothing about the existing rows will change

如果我将表修改为设置一个默认值，那么关于现有行的任何内容都不会改变

SQL> alter table foo
  2    modify( col2 varchar2(10) default 'foo' );

Table altered.

SQL> select * from foo;

      COL1 COL2
---------- ----------
         1
         2
         3

SQL> insert into foo( col1 ) values (4);

1 row created.

SQL> select * from foo;

      COL1 COL2
---------- ----------
         1
         2
         3
         4 foo

Even if I subsequently change the default again, there will still be no change to the existing rows

即使我随后再次更改默认值，对现有行仍然没有更改

SQL> alter table foo
  2    modify( col2 varchar2(10) default 'bar' );

Table altered.

SQL> select * from foo;

      COL1 COL2
---------- ----------
         1
         2
         3
         4 foo

SQL> insert into foo( col1 ) values (5);

1 row created.

SQL> select * from foo;

      COL1 COL2
---------- ----------
         1
         2
         3
         4 foo
         5 bar

#2

ALTER TABLE {TABLE NAME}
ALTER COLUMN {COLUMN NAME} SET DEFAULT '{DEFAULT VALUES}'

example :

例子:

ALTER TABLE RESULT
ALTER COLUMN STATUS SET DEFAULT 'FAIL'

#3

Following Justin's example, the command below works in Postgres:

以Justin为例，下面的命令在Postgres中工作:

alter table foo alter column col2 set default 'bar';

更改表foo alter列col2设置默认的'bar';

#4

-1

For Sql Azure the following query works :

对于Sql Azure，下面的查询工作如下:

ALTER TABLE [TableName] ADD  DEFAULT 'DefaultValue' FOR ColumnName
GO

#1