SQL DEFAULT 约束
SQL DEFAULT 约束
The 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'
);The 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';删除 DEFAULT 约束
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;
