Seven constraint

Constraints are used to limit the types of added data table.

Constraints can be defined (by when you create a table CREATE TABLE statement), or after the table can also be created (via ALTER TABLE statement).

. 1, the NOT NULL : NULL constraint force does not accept the column value.

２、UNIQUE：

UNIQUE constraint uniquely identifies each record in the database table.

UNIQUE and PRIMARY KEY constraints are column or set of columns to provide a guarantee of uniqueness.

PRIMARY KEY constraint automatically has a UNIQUE definition.

Note that each table can have multiple UNIQUE constraints, but each table can have only one PRIMARY KEY constraint.

MySQL：

CREATE TABLE Persons
(
Id_P int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
UNIQUE (Id_P)
)

SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
Id_P int NOT NULL UNIQUE,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

ALTER  TABLE Persons
 ADD  UNIQUE (Id_P)

３、PRIMARY KEY:

PRIMARY KEY constraint uniquely identifies each record in the database table.

Primary keys must contain unique values.

Primary key column can not contain a NULL value.

Each table should have a primary key, and each table can have only one primary key.

MySQL：

CREATE TABLE Persons
(
Id_P int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
PRIMARY KEY (Id_P)
)

SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
Id_P int NOT NULL PRIMARY KEY,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

ALTER TABLE Persons
ADD PRIMARY KEY (Id_P)

４、FOREIGN KEY:

A table pointing FOREIGN KEY PRIMARY KEY in another table.

MySQL：

CREATE TABLE Orders
(
Id_O int  NOT  NULL ,
OrderNo int NOT NULL,
Id_P int,
PRIMARY KEY (Id_O),
FOREIGN KEY (Id_P) REFERENCES Persons(Id_P)
)

SQL Server / Oracle / MS Access:

CREATE TABLE Orders
(
Id_O int NOT NULL PRIMARY KEY,
OrderNo int NOT NULL,
Id_P int FOREIGN KEY REFERENCES Persons(Id_P)
)

ALTER TABLE Orders
ADD FOREIGN KEY (Id_P)
REFERENCES Persons(Id_P)

Revocation of constraints:

ALTER  TABLE Orders
 DROP  FOREIGN  KEY fk_PerOrders #MySQL

ALTER TABLE Orders
DROP CONSTRAINT fk_PerOrders  #SQL Server / Oracle / MS Access:

５、CHECK :

CHECK constraints for limiting the range of values ​​in the column.

If CHECK constraints on the definition of a single column, then the value of the specific column only.

If the definition CHECK constraints on a table, then the constraint will limit the value of a particular column.

My SQL:  CHECK(ID_P>0)

　　　　  ALTER TABLE Persons ADD CHECK (Id_P>0)

　　　　  ALTER TABLE Persons DROP CHECK ID_P #取消约束

SQL Server / Oracle / MS Access:  ID_P int not null CHECK(ID_P>0)

　　　　  ALTER TABLE Persons DROP CONSTRAINT ID_P

６、DEFAULT :

DEFAULT constraint for inserting a default value to the column.

If no other value is specified, then the default value will be added to all new records.

CREATE TABLE Orders
(
Id_O int  NOT  NULL ,
OrderNo int NOT NULL,
Id_P int,
OrderDate date DEFAULT GETDATE()
)

MySQL:

ALTER TABLE Persons
ALTER City SET DEFAULT 'SANDNES'

ALTER TABLE Persons
ALTER City DROP DEFAULT

SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ALTER COLUMN City SET DEFAULT 'SANDNES'

ALTER TABLE Persons
ALTER COLUMN City DROP DEFAULT