SQL 默认 Constraint
SQL DEFAULT Constraint
这 DEFAULT constraint is used to set a default value for a column.
The default value will be added to all new records if no other value is specified.
SQL DEFAULT on CREATE TABLE
The following SQL sets a DEFAULT value for the "City" column when the "Persons" table is created:
My SQL / SQL Server / Oracle / MS Access:
CREATE TABLE Persons (
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int,
City varchar(255) DEFAULT 'Sandnes'
);
这 DEFAULT constraint can also be used to insert system values, by using functions like GETDATE():
CREATE TABLE Orders (
ID int NOT NULL,
OrderNumber int NOT NULL,
OrderDate date DEFAULT GETDATE()
);
SQL DEFAULT on ALTER TABLE
To create a DEFAULT constraint on the "City" column when the table is already created, use the following SQL:
MySQL:
ALTER TABLE Persons
ALTER City SET DEFAULT 'Sandnes';
SQL Server:
ALTER TABLE Persons
ADD CONSTRAINT df_City
DEFAULT 'Sandnes' FOR City;
MS Access:
ALTER TABLE Persons
ALTER COLUMN City SET DEFAULT 'Sandnes';
Oracle:
ALTER TABLE Persons
MODIFY City DEFAULT 'Sandnes';
DROP a DEFAULT Constraint
To drop a DEFAULT constraint, use the following SQL:
MySQL:
ALTER TABLE Persons
ALTER City DROP DEFAULT;
SQL Server / Oracle / MS Access:
ALTER TABLE Persons
ALTER COLUMN City DROP DEFAULT;
SQL Server:
ALTER TABLE Persons
ALTER COLUMN City DROP DEFAULT;
