foreign key references

mysql add foreign key

-- On Create
CREATE TABLE tableName (
    ID INT,
    SomeEntityID INT,
    PRIMARY KEY (ID),
    FOREIGN KEY (SomeEntityID)
        REFERENCES SomeEntityTable(ID)
        ON DELETE CASCADE
);

-- On Alter, if the column already exists but has no FK
ALTER TABLE
  tableName
ADD
  FOREIGN KEY (SomeEntityID) REFERENCES SomeEntityTable(ID) ON DELETE CASCADE;
  
 -- Add FK with a specific name
 -- On Alter, if the column already exists but has no FK
ALTER TABLE
  tableName
ADD CONSTRAINT fk_name
  FOREIGN KEY (SomeEntityID) REFERENCES SomeEntityTable(ID) ON DELETE CASCADE;

Posted by: Guest on April-27-2020

CREATE TABLE Employee( EmployeeID int NOT NULL, LastName varchar(50) NOT NULL, FirstName varchar(20) NOT NULL, Age int, DeptNo int, PRIMARY KEY (EmployeeID), FOREIGN KEY (DeptNo) REFERENCES Department(DeptNo) );

Foreign Key: It is a column that comes from a different table and using Foreign key tables are related each other It is the primary key of another table It can be duplicate or null for another table Primary Key : It is unique column in every table in a database It can ONLY accept; - nonduplicate values - cannot be NULL Unique Key: Only unique value and also can contain NULL

USE Organization CREATE TABLE Employee ( Id INT PRIMARY KEY IDENTITY(1,1), Name VARCHAR (50) NOT NULL, Age INT, Gender VARCHAR (50), Dep_Id int FOREIGN KEY REFERENCES Department(Id), Insur_Id int FOREIGN KEY REFERENCES Insurance(Id) )

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