- Sql Server Sequence Generated Primary Key On Multiple Columns In Excel
- Sql Server Sequence Generated Primary Key On Multiple Columns Examples
- Sql Alter Column Primary Key
Summary: in this tutorial, you will learn how to use Oracle PRIMARY KEY constraint to manage the primary key of a table.
APPLIES TO: SQL Server Azure SQL Database Azure Synapse Analytics (SQL DW) Parallel Data Warehouse. Creates a sequence object and specifies its properties. A sequence is a user-defined schema bound object that generates a sequence of numeric values according to the specification with which the sequence was created. Surrogate keys are keys that have no “business” meaning and are solely used to identify a record in the table. Such keys are either database generated (example: Identity in SQL Server, Sequence in Oracle, Sequence/Identity in DB2 UDB etc.) or system generated values (like generated via a table in the schema).
Introduction to the primary key
A primary key is a column of a combination of columns in a table that uniquely identifies a row in the table.
A sequence is an object in SQL Server (Transact-SQL) that is used to generate a number sequence. This can be useful when you need to create a unique number to act as a primary key. Create Sequence.
The following are rules that make a column a primary key:
- A primary key column cannot contain a NULL value or an empty string.
- A primary key value must be unique within the entire table.
- A primary key value should not be changed over time.
According to these rules, the following are the recommendations for the primary keys:
- First, the primary key should be meaningless. Sometimes, you may want use meaningful data, which considers being unique, for the primary keys e.g., social security number (SSN), vehicle identification number (VIN), email, and phone number. However, you don’t know when the email or phone number changes or is reused by another person. In such cases, it will create many data problems. In the database world, the artificial keys are known as surrogate keys which are as opposed to natural primary keys.
- Second, the primary keys should be compact. The primary keys typically are numeric because Oracle typically processes numbers faster than any other data types.
To create a primary key in a table, you use the PRIMARY KEY constraint.
Oracle PRIMARY KEY constraint examples
Typically, you create a primary key for a table when you create that table. In addition, you can add a primary key to a table after the fact by using the ALTER TABLE statement.
Creating a primary key that consists of one column
The following CREATE TABLE statement creates the purchase_orderstable:
The purchase_orders table has four columns purchase order number (po_nr), vendor id (vendor_id), purchase order status (po_status), and the timestamp (created_at) of which the purchase order is created.
In this table, defined the po_nr column as the primary key by using the PRIMARY KEY clause.
Note that the PRIMARY KEY clause implicitly makes the po_nrcolumn NOT NULL so you don’t have to define the column like:
The PRIMARY KEY constraint in this example is an inline constraint because it is on the same line as the po_nr column.
Consider the following statement.
This example used the PRIMARY KEY constraint as the table constraint. Notice the following clause:
In addition, we explicitly assigned the PRIMARY KEY constraint a name pk_purchase_orders.
Creating a primary key that consists of multiple columns
Sql Server Sequence Generated Primary Key On Multiple Columns In Excel
The following statement creates the purchase order line items table:
In this example, the primary key of the purchase_order_items table consists of two columns: po_nr and item_nr. It means that the combination of values of these columns uniquely identifies a purchase order line item.
This example did not use the CONSTRAINT clause to explicitly assign the PRIMARY KEY constraint a name. Therefore, Oracle implicitly assigned the primary key constraint a system-generated name such as SYS_C0010617.
Adding a primary key to a table
Sometimes, you may want to add a primary key constraint to an existing table. To do it, you use the ALTER TABLE statement as follows:
The following example creates the vendors table first and then adds a primary key constraint to it:
Dropping an Oracle PRIMARY KEY constraint
You will rarely drop a PRIMARY KEY constraint from a table. If you have to do so, you use the following ALTER TABLE statement.
For example, you can drop the primary key constraint of the vendors table as follows:
It is possible to use the following statement to drop the primary key of a table:
For example:
Enable / Disable an Oracle PRIMARY KEY constraint
To improve the performance when loading a large amount of data into a table or updating mass data, you can temporarily disable the PRIMARY KEY constraint.
To disable a PRIMARY KEYconstraint of a table, you use the ALTER TABLE statement:
or
For example, to disable the primary key constraint of the purchase_orders table, you use the following statement:
or
To enable a primary key constraint, you use the following ALTER TABLE statement:
or
The following example enables the PRIMARY KEY constraint of the purchase_orders table:
or
In this tutorial, you have learned how to use Oracle PRIMARY KEY constraint to create, add, disable, enable, and drop a primary key of a table.
Summary: in this tutorial, you will learn how to use the Db2 PRIMARY KEY constraint to define a primary key for a table.
Db2 primary key overview
A primary key of a table is a column or group of columns whose values uniquely identify every row in the table.
Each table has one and only one primary key. A primary key is optional. However, it’s a good practice to have a primary key in every table.
To define a primary key for a table, you use the PRIMARY KEY constraint.
Creating a table with a primary key
The following statement creates a table with a primary key:
If the primary key consists of two or more columns, you use the following syntax:
When you create a table with the primary key constraint, Db2 automatically creates a unique index on the primary key columns. This unique index is also known as the primary index.
Adding a primary key to an existing table
To add a primary key to a table, you use the following form of ALTER TABLE statement:
Note that the ADD CONSTRAINT clause is optional.
Removing a primary key from a table
To remove a primary key from a table, you use the following form of the ALTER TABLE statement:
Primary key and NOT NULL constraint
The primary key of a table cannot contain NULL values. In other database systems such as SQL Server, Oracle, and MySQL, the columns included in the primary key columns will implicitly receive the NOT NULL constraint. However, in DB2, you must explicitly declare all primary key columns as NOT NULL columns.
Primary key and identity column
Because the primary key must contain unique values that uniquely identify each row of a table, the identity column is an ideal choice for the primary key.
The following syntax shows how to create a table with a primary key as an identity column:
Db2 primary key examples
Let’s take some examples of creating new tables with primary keys.
1) Creating a table with a primary key example
Suppose SSN uniquely identifies a person. The following statement creates the persons table whose primary key is the SSN column:
This statement inserts a new person into the persons table:
The following statement attempts to insert a new row into the persons table with an SSN that already exists:
Db2 issued the following error:
Sql Server Sequence Generated Primary Key On Multiple Columns Examples
2) Creating a table with a primary key as an identity column example
The following example creates a table named members with a primary key column as an identity column:
Each time you insert a new row into the members table, Db2 automatically generates a sequential number for the member_id column.
Sql Alter Column Primary Key
Here are the contents of the members table:
Notice that the member_id has the sequential integer 1 and 2.
3) Creating a new table whose primary key has more than one column example
The following example creates a new table whose primary key contains two columns:
4) Adding a primary key to an existing table example
Windows 7 product key generator. This example creates a new table without a primary key:
Before promoting the membership_id as the primary key, you first need to make sure that this column does not have duplicate values. Then, you use the following statement to add a primary key to the memberships table:
In this tutorial, you have learned about Db2 primary key and how to create a new table with a primary key.