Default constraint in database

[Aileen_L]

Hello sir,
I am creating a table and need to specify a column as a TRUE or FALSE values but Unable to find any concept regrading this.I tried it from book also.I need to know in detailed about this.I am excited to implement it and need your help.

Thanks..

[Praetor]

Default constraint :

The default constraint is very useful constraint to insert the default value into the column but there is no any concept of Boolean values in the database.It is applied for the database parameter which is performed by DBMS itself.

You can implement the Boolean values according to your way with the help of Default constraint.

[Katty]

Specifying Default constraint :

You can specify the default constraint for table column after and before creation of the table.Add the Default option with column during creation and using alter command you can add it later.

This is the syntax to implement the default constraint for a database :

CREATE TABLE Persons
(
Id int NOT NULL,
Name varchar(255) NOT NULL,
City varchar(255),
Sex DEFAULT 'M'
)

[kelfro]

DEFAULT Constraint on ALTER TABLE:

You can ALTER the table of database and implement the DEFAULT constraint.To do this you need to alter table first and then column with the combined statement.Use this statement to perform this task :

ALTER TABLE Persons
ALTER COLUMN City SET DEFAULT 'Washington';

The given example shows that a constraint has been added with City column and the values "Washington" would be inserted automatically when you skip the column to insert.

[Zecho]

Dropping the DEFAULT Constraint :

You can drop the default constraint after implementing.The column would be free from any restriction of constraint.Use following SQL statement to drop it :

ALTER TABLE Persons
ALTER COLUMN City DROP DEFAULT