alter table allow null

Null Value Null indicates that the value is unknown. The null value is different from the blank or zero value. There are no two equal null values. Compare two null values or compare null values with any other values to return unk

Before testing and use of the time really did not find this problem, always think that copy_table_dependents will automatically filter not NULL constraints. However, the fact is that, if you intend to use the Copy_table_dependents procedure to replicate related objects such as indexes, constraints, and permissions, the existence of a NOT NULL constraint raises an error when the target

First take a look at the simplest example, in test, add a field, the field name is birth, the type is date type. Mysql> ALTER TABLE test add column Birth date;Query OK, 0 rows affected (0.36 sec)records:0 duplicates:0 warnings:0 Check the data and see the results: Mysql> select * from test;+------+--------+----------------------------------+------------+-------+| t_id | T_name | T_password | T_birth | Birth

recommended to use the default NULLA string of type can default ' int ' type can default 0Landlord want to determine whether the field is empty with PHP processing can Sql:using NULL values vs. default valuesIs it good to use Default:null?No default value vs NULL vs 0 in MySQL as a default value for text and integer fields First of all, 3 of the stackoverflow discussion on the issue, in fact, I personall

UNIQUE constraint in the "id_p" column when the "Persons" table is created:Mysql:CREATE TABLE Persons (id_p int not null,lastname varchar (255) not null,firstname varchar (255), Address varchar (255), City VA Rchar (255), UNIQUE (Id_P) )SQL Server/oracle/ms Access:UNIQUE, LastName varchar (255) not

Alter TABLE table1 ALTER COLUMN [name] varchar () NULL; Table1 Table Name Name field name why add [], because name is the SQL keyword will conflict error, so just in case. If the name field has already created an index, you must delete the index before you can correct it if

into it, the table will not immediately allocate extent, that is, it does not occupy the data space, only when you insert the data to allocate space. This can save a little space. When the Deferred_segment_creation parameter is set to False, both the empty table and the non-empty table are assigned segment. It is important to note that this value is set only aft

-- Query table structure information-- Maidan's (1)SelectTable name = case when a. colorder = 1 then D. Name else ''end,Table description = case when a. colorder = 1 then isnull (F. Value, '') else'' end,FIELD No. = A. colorder,Field name = A. Name,Id = case when columnproperty (A. ID, A. Name, 'isidentity ') = 1 then' √ 'else' end,Primary Key = case when exists (select 1 from sysobjects where xtype = 'pk'