what is the meaning of foreign key and use

Question

MuthuMari M · Answer

Hi selvi A foreign key is a field (or fields) that points to the primary key of another table. The purpose of the foreign key is to ensure referential integrity of the data. In other words only values that are supposed to appear in the database are permitted. Pls refer this url for examples http://www.1keydata.com/sql/sql-foreign-key.html Thanks.

lakshmipathi vemula · Answer

Hello Friend You have 2 tables and you want to insert data into a table such that one column in that table should have the values which are there in the other table with same column datatype(Here not same number of values but same values..other values which are not there in another table for which this Foreign key is mapped with Primary key should not be entered).This is like setting range for values to be entered into some column in a table.. For each foreign key defined in one table .there should be primary key set in another table. To know more about SQL http://www.bestdotnettraining.com/Online/Training/SQL-Server/Constraints/152

Senthilkumar · Answer

Hi The foreign key is key which has the reference in the parent table. A foreign key (FK) is a column or combination of columns used to establish and enforce a link between the data in two tables. A link is created between two tables by adding the column or columns that hold one table's primary key values to the other table. This column becomes a foreign key in the second table. You can create a foreign key by defining a FOREIGN KEY constraint when you create or alter a table. Why we need to use the foreign key in the table? It ensures that there is an integrity between the tables. For example you have the table called EmployeeMaster. The employee id is the primary key and it is eligible to became the foreign in the child tables. If any body try to enter the salary for unknown employee then its error. Employee Table: EmployeeId Name DOB DOJ 1 S 1/1/1900 5/6/2010 2 K 5/7/1989 4/4/2011 EmployeeID is PRIMARY KEY it ensures here it will not allow the duplicate entries and creates the clustered index. Salary Table: ID EmployeeID Month Year Salary 1 1 5 2012 24500 2 2 5 2012 15000 3 3 5 2012 35000 Here EmployeeID is foreign key it has the relationship withe Employees table. Here i try to insert the record for Employee ID 3 but it is an not relevant. It throws an error while inserting.

Keyur Narkhi · Answer

Hi Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable{mso-style-name:Table Normal;mso-tstyle-rowband-size:0;mso-tstyle-colband-size:0;mso-style-noshow:yes;mso-style-priority:99;mso-style-qformat:yes;mso-style-parent:;mso-padding-alt:0in 5.4pt 0in 5.4pt;mso-para-margin-top:0in;mso-para-margin-right:0in;mso-para-margin-bottom:10.0pt;mso-para-margin-left:0in;line-height:115%;mso-pagination:widow-orphan;font-size:11.0pt;font-family:Calibrisans-serif;mso-ascii-font-family:Calibri;mso-ascii-theme-font:minor-latin;mso-fareast-font-family:Times New Roman;mso-fareast-theme-font:minor-fareast;mso-hansi-font-family:Calibri;mso-hansi-theme-font:minor-latin;mso-bidi-font-family:Times New Roman;mso-bidi-theme-font:minor-bidi;} A foreign key (FK) is a column or combination of columns that is used to establish and enforce a link between the data in two tables. You can create a foreign key by defining a FOREIGN KEY constraint when you create or modify a table. Below is the Example for FK. Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable{mso-style-name:Table Normal;mso-tstyle-rowband-size:0;mso-tstyle-colband-size:0;mso-style-noshow:yes;mso-style-priority:99;mso-style-qformat:yes;mso-style-parent:;mso-padding-alt:0in 5.4pt 0in 5.4pt;mso-para-margin-top:0in;mso-para-margin-right:0in;mso-para-margin-bottom:10.0pt;mso-para-margin-left:0in;line-height:115%;mso-pagination:widow-orphan;font-size:11.0pt;font-family:Calibrisans-serif;mso-ascii-font-family:Calibri;mso-ascii-theme-font:minor-latin;mso-fareast-font-family:Times New Roman;mso-fareast-theme-font:minor-fareast;mso-hansi-font-family:Calibri;mso-hansi-theme-font:minor-latin;mso-bidi-font-family:Times New Roman;mso-bidi-theme-font:minor-bidi;} Table Customer column name characteristic SID Primary Key Last_Name First_Name Table Orders column name characteristic Order_ID Primary Key Order_Date Customer_SID Foreign Key Amount Here are two tables one is Customer and other is Orders table.So Customer table has primary key which reference into the Orders table.So Customer_SID is the FK. Thanks

selvi subramanian · Answer

Like dis satyapriya.

Satyapriya Nayak · Answer

Hi Selvi A FOREIGN KEY in one table points to a PRIMARY KEY in another table. Please refer the below link http://www.w3schools.com/sql/sql_foreignkey.asp Thanks