alter table allow null

Alter table tablename add (column datatype [default value] [null/not null],...); Alter table tablename modify (column datatype [default value] [null/not

The MySQL ALTER TABLE statement can modify the basic structure of the table, such as adding fields, deleting fields, adding primary keys, adding indexes, modifying field data types, renaming tables, and so on, and this article introduces the use of MySQL ALTER TABLE with two

, therefore, the entire modification operation is maintained for a long time. Let's perform a test: -- ====================================-- Use the test database db2use db2go -- ========== =======-- create test table create table test001 (C1 int identity ), c2 bigint) Go -- =======================-- import 2000 + rows of data into the test table insert into tes

Alter table only allows adding columns that can contain null values or specified the default definition. If: If exists (select 1 from sysobjects where name = n' tab _ test') and not exists (select 1 from sysobjects A, syscolumns B where B. id =. ID and B. name = n' Col _ test' and. name = n' tab _ test ')Alter

SQL ALTER TABLE syntax and example tutorial After the alter table statement allows you to change the structure of a TABLE, it is created. Address terms that can be added to a new column. Existing columns may be deleted from the Drop clause

test where ID mysql> TRUNCATE TABLE test; #删除allMysql> select * from test;Empty Set (0.00 sec) On top, modify the table name in the database, add and remove fields from the table. mysql> Use GBK #进入gbk数据库mysql> desc test;+-------+----------+------+-----+---------+----------------+| Field | Type | Null | Key | Defaul

SQL ALTER TABLE Syntax and instance tutorial The ALTER TABLE statement allows you to change the structure of a table after it has been created. New column can be added to the address clause. Existing columns may be deleted from the list of drop clauses used. The syntax for

DB2 alter: Add/delete/Reset Column 1. Add Fields Alter table [table_name] add [column_name] [column_type] 2. Change the field type Alter table [table_name] alter column [column_name] Set Data Type [column_type] Note: There ar

, because they cannot be used for null queries, therefore, to avoid null in fields frequently to be queried, you can use default to process them. However, if "unknown" cannot be replaced by any default value, it can only be identified by null. For example, the value of a field may be negative infinity to positive infinity. So what default will you use to identify

. The new field is always added to the end of the existing field list. Column-def can be any form that is allowed in create table and is subject to the following restrictions: A field cannot have a primary key or a unique constraint. Fields cannot have these default values: Current_time, current_date, or Current_timestamp If a NOT NULL constraint is defined, the field must have a non-empty

Oracle Constraints * If a constraint only applies to individual fields, you can define the constraint at the field level or at the table level. However, if a constraint acts on multiple fields,Constraints must be defined at the table level* When defining a constraint, you can use the constraint keyword to name the constraint. If not specified, oracle automatically creates a default name for the constraint.

Oracle alter table, oraclealtertable Create table T_CLASS_INFO (CLASSNO number (3) primary key, -- class numberCLASSNAME varchar2 (10 ),CLASSPLACE varchar2 (13));Create table T_STUDENT_INFO (STUNO number (3), -- Student IDCLASSNO number (3 ),STUNAME varchar2 (10 ),STUSEX char (1 ),STUCONTECT number (11), -- contact inf

The following error was reported when modifying the auto-grow value of the table's primary key:mysql> ALTER TABLE YOON change COLUMN ID ID INT (one) not NULL auto_increment ADD PRIMARY KEY (ID);Error 1064 (42000): You have a error in your SQL syntax; Check the manual-corresponds to your MySQL server version for the right syntax-use-near ' ADD-PRIMARY KEY (ID) ' a