Generate Unique Key In Sql
SQL FOREIGN KEY Constraint
'We have around 50 tables in the our database which are added by our installer but we do not have any primary key. So we need to generate add a column in each table as that tables primary key and generate that with unique primary key.' Absolutely not! Don't add a surrogate key to every table. In SQL Server the unique key has the following characteristics: There can be multiple unique keys defined on a table. Unique Keys result in NONCLUSTERED Unique Indexes by default. One or more columns make up a unique key. Column may be NULL, but on one NULL per column is allowed. A unique constraint can be referenced by a Foreign Key Constraint. Apr 20, 2006 How to Generate Sequences and Surrogate Keys in Generic SQL. Which uniquely identifies every row in the table and is “more minimal” than the inherently unique aspect of the data. The usual choice is a monotonically increasing integer, which is small and easy to use in foreign keys. It’s possible to generate the value in SQL, but it. There is one other item to note. SQL Server keeps the last generated identity value in memory which can be retrieved right after an INSERT using SCOPEIDENTITY, @@IDENTITY, or CHECKIDENT (depending on the scope you require). There is nothing similar to capture the last generated GUID value. Unique Key in SQL A unique key is a set of one or more than one fields/columns of a table that uniquely identify a record in a database table. You can say that it is little like primary key but it can accept only one null value and it cannot have duplicate values.
Oracle / PLSQL: Unique Constraints This Oracle tutorial explains how to create, drop, disable, and enable unique constraints in Oracle with syntax and examples. What is a unique constraint in Oracle? A unique constraint is a single field or combination of fields that uniquely defines a record. SQL Server contains the NEWID function. This function creates a unique value of type uniqueidentifier. We can use this function in several ways to generate unique numbers to suit our requirements: NEWID generate alphanumeric value of 36 char.
A FOREIGN KEY is a key used to link two tables together.
A FOREIGN KEY is a field (or collection of fields) in one table that refers to the PRIMARY KEY in another table.
The table containing the foreign key is called the child table, and the table containing the candidate key is called the referenced or parent table.
Look at the following two tables:
'Persons' table:
| PersonID | LastName | FirstName | Age |
|---|---|---|---|
| 1 | Hansen | Ola | 30 |
| 2 | Svendson | Tove | 23 |
| 3 | Pettersen | Kari | 20 |
'Orders' table:
| OrderID | OrderNumber | PersonID |
|---|---|---|
| 1 | 77895 | 3 |
| 2 | 44678 | 3 |
| 3 | 22456 | 2 |
| 4 | 24562 | 1 |
Notice that the 'PersonID' column in the 'Orders' table points to the 'PersonID' column in the 'Persons' table.
The 'PersonID' column in the 'Persons' table is the PRIMARY KEY in the 'Persons' table.
The 'PersonID' column in the 'Orders' table is a FOREIGN KEY in the 'Orders' table.
The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables.
The FOREIGN KEY constraint also prevents invalid data from being inserted into the foreign key column, because it has to be one of the values contained in the table it points to.
SQL FOREIGN KEY on CREATE TABLE
The following SQL creates a FOREIGN KEY on the 'PersonID' column when the 'Orders' table is created:
MySQL:
OrderID int NOT NULL,
OrderNumber int NOT NULL,
PersonID int,
PRIMARY KEY (OrderID),
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)
);
SQL Server / Oracle / MS Access:
OrderID int NOT NULL PRIMARY KEY,
OrderNumber int NOT NULL,
PersonID int FOREIGN KEY REFERENCES Persons(PersonID)
);
To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
OrderID int NOT NULL,
OrderNumber int NOT NULL,
PersonID int,
PRIMARY KEY (OrderID),
CONSTRAINT FK_PersonOrder FOREIGN KEY (PersonID)
REFERENCES Persons(PersonID)
);
SQL FOREIGN KEY on ALTER TABLE
To create a FOREIGN KEY constraint on the 'PersonID' column when the 'Orders' table is already created, use the following SQL:
MySQL / SQL Server / Oracle / MS Access:
ADD FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);
Generate Unique Key In Sql Excel
To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
ADD CONSTRAINT FK_PersonOrder
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);
DROP a FOREIGN KEY Constraint
To drop a FOREIGN KEY constraint, use the following SQL:
MySQL:
DROP FOREIGN KEY FK_PersonOrder;
SQL Server / Oracle / MS Access:
Oracle Unique Key
DROP CONSTRAINT FK_PersonOrder;