SQL FOREIGN KEY

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In a watch FOREIGH KEY Point to... In another table PRIMARY KEY.

Explain foreign keys through examples . Look at the following two tables ：

Be careful :

·”Orders” In the table ”P_Id” The column points to ”Persons” In the table ”P_Id” Column .

·”Persons” In the table ”P_Id” The column is ”Persons” In the table PRIMARY KEY.

·”Orders” In the table ”P_Id” The column is ”Orders” In the table FOREIGN KEY.

FOREIGN KEY Constraints are used to prevent the behavior of breaking connections between tables .

FOREIGN KEY Constraints also prevent illegal data from being inserted into foreign key columns , Because it has to be one of the values in the table it points to .

CREATE TABLE At the time of the SQL FOREIGN KEY constraint

stay ”Orders” The table is created at ”P_Id” Column to create FOREIGN KEY constraint ：

MySQL：

CREATE TABLE Orders
(
O_Id int NOT NULL,
OrderNo int NOT NULL,
P_Id int,
PRIMARY KEY (O_Id),
FOREIGN KEY (P_Id) REFERENCES Persons(P_Id)
)

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SQL Server/ Oracle /MS Access:

CREATE TABLE Orders
(
O_Id int NOT NULL PRIMARY KEY,
OrderNo int NOT NULL,
P_Id int FOREIGN KEY REFERENCES Persons(P_Id)
)

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To name FOREIGN KEY constraint , And define multiple columns of FOREIGN KEY constraint , Please use the following SQL grammar :

MySQL /SQL Server /Oracle /MS Access:

CREATE TABLE Orders
(
O_Id int NOT NULL,
OrderNo int NOT NULL,
P_Id int,
PRIMARY KEY (O_Id),
CONSTRAINT fk_PerOrders FOREIGN KEY (P_Id)
REFERENCES Persons(P_Id)
)

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ALTER TABLE At the time of the SQLFOREIGN KEY constraint

When ”Orders” When the table has been created , If you need to be in ”P_Id” Column creation FOREIGN KEY constraint , Please use the following SQL:

MySQL /SQL Server /Oracle /MS Access

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

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To name FOREIGN KEY constraint , And define multiple columns of FOREIGN KEY constraint , Please use the following SQL grammar ：

MySQL /SQL Server /Oracle /MS Access:

ALTER TABLE Orders
ADD CONSTRAINT fk_PerOrders
FOREIGN KEY (P_Id)
REFERENCES Persons(P_Id)

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revoke FOREIGN KEY constraint If you need to cancel FOREIGN KEY constraint , Please use the following SQL:

MySQL:

ALTER TABLE Orders
DROP FOREIGN KEY fk_PerOrders

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SQL Server /Oracle /MS Access:

ALTER TABLE Orders
DROP CONSTRAINT fk_PerOrders

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Reference resources ：

https://www.yuque.com/docs/share/79273585-3f48-4505-b43b-edb59ba84662

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