SQL中 ALTER TABLE 只允许添加可包含空值或指定了 DEFAULT 定义的列。

ALTER TABLE 只允许添加可包含空值或指定了 DEFAULT 定义的列。

　　如果：

IF EXISTS(SELECT 1 FROM SYSOBJECTS WHERE NAME = N'tab_test') AND NOT EXISTS(SELECT 1 FROM SYSOBJECTS A, SYSCOLUMNS B WHERE B.ID = A.ID AND B.NAME = N'col_test' and a.name = N'tab_test')
alter table tab_test
add col_test smallint NOT NULL
GO

　　将会收到错误信息。

　　要增加非空列怎么办呢？

　　第1种办法：在add column时指定列有default

alter table tab_test
add col_test NOT NULL CONSTRAINT DFtab_test_col_test DEFAULT 1

　　第2种办法：先给表增加个table_constraint default,再alter column

alter table tab_test
add CONSTRAINT DFtab_test_col_test DEFAULT 1 for col_testalter table tab_test
alter column col_test int NOT NULL

　　第3种办法：只好先增加空列，在修改为非空啦：

IF EXISTS(SELECT 1 FROM SYSOBJECTS WHERE NAME = N'tab_test') AND NOT EXISTS(SELECT 1 FROM SYSOBJECTS A, SYSCOLUMNS B WHERE B.ID = A.ID AND B.NAME = N'col_test' and a.name = N'tab_test')
alter table tab_test
add col_test smallint
GO
alter table tab_test
alter column col_test smallint not null
GO

语法如下：

ALTER TABLE table_name
    ALTER COLUMN column_name 
   {
    type_name[({precision[.scale]})][NULL|NOT NULL]
   {DROP DEFAULT 
   | SET DEFAULT constant_expression 
   | IDENTITY [ ( seed , increment ) ]
   } 
| ADD 
   { < column_definition > | < table_constraint > } [ ,...n ] 
| DROP 
   { [ CONSTRAINT ] constraint_name 
   | COLUMN column }
] }
< column_definition > ::= 
   { column_name data_type } 
   [ [ DEFAULT constant_expression ] 
      | IDENTITY [ ( seed , increment ) ] 
   ] 
   [ROWGUIDCOL]
   [ < column_constraint > ] [ ...n ] ]
< column_constraint > ::= 
   [ NULL | NOT NULL ] 
   [ CONSTRAINT constraint_name ] 
   { 
      | { PRIMARY KEY | UNIQUE } 
      | REFERENCES ref_table [ (ref_column) ] 
      [ ON DELETE { CASCADE | NO ACTION | SET DEFAULT |SET NULL } ] 
      [ ON UPDATE { CASCADE | NO ACTION | SET DEFAULT |SET NULL } ]
   }
< table_constraint > ::= 
   [ CONSTRAINT constraint_name ] 
   { [ { PRIMARY KEY | UNIQUE } 
      { ( column [ ,...n ] ) } 
      | FOREIGN KEY 
        ( column [ ,...n ] )
        REFERENCES ref_table [ (ref_column [ ,...n ] ) ] 
      [ ON DELETE { CASCADE | NO ACTION | SET DEFAULT |SET NULL } ] 
      [ ON UPDATE { CASCADE | NO ACTION | SET DEFAULT |SET NULL } ] 
   }