When creating the primary key for a table, we might need to look at more than a single column. A primary key can consist of several attributes that together provide a unique identifier for our records, called “composite key”. In this article, we will learn about composite primary keys and how to create them in SQL Server.
Composite primary key
Primary keys are special types of constraint that uniquely identify all rows in a table. Usually, we choose a single column as the primary key in our table to maintain data integrity. Adobe acrobat dc pro key generator. However, if necessary, we can also choose several columns as a primary key to prevent duplications. Dmg mori parts.
Often, composite keys are useful when we’re working with relation tables, which are tables that connect two or more tables.
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Create Primary Keys.; 2 minutes to read +6; In this article. APPLIES TO: SQL Server 2016 and later Azure SQL Database Azure Synapse Analytics (SQL DW) Parallel Data Warehouse You can define a primary key in SQL Server by using SQL Server Management Studio or Transact-SQL. Apr 22, 2010 ALTER TABLE dbo.TestTable ADD CONSTRAINT PKTestTable PRIMARY KEY CLUSTERED (ID ASC) GO. He wanted to know if we can create Primary Key as part of the table name as well, and also give it a name at the same time. Though it would look very normal to all experienced developers, it can be still confusing to many.
Let’s look at an example of a composite primary key.
In a simple hotel reservation system, suppose that each person only stays for one day in the hotel. Audials tunebite license key generator. We wouldn’t want to have a room reserved by two guests on the same night. By creating a composite key that contains the date and room number, we can ensure that a room is not double reserved for the same day.
See the following table for an illustration.
Table 1. Sample table that stores hotel reservation data Free download game g t vice city.
From Table 1, we can see that the table can have the same room on different dates, and many rooms on the same day. However, the database will not allow two rows to have the same combination of dates and room numbers.
How to create composite keys in SQL Server
Now, let’s see demonstrations on how to create a composite key using SQL Server Management Studio (SSMS) and T-SQL.
As an example case, let’s use the simple hotel reservation system we discussed previously. We will create a table named Reservation with three columns: RoomNumber, DateOfStay, and Guest. Then, we’re going to make the RoomNumber and DateOfStay columns as a composite key in this table.
We’ll take a look at the steps below.
Using Management Studio
Step 1. In the Object Explorer, select the Tables folder under the database. Then, right-click and select New > Table.
Sql Server Auto Increment Primary Key Management Studio
Figure 1. Opening the table editor in the design view Free alternative to spotify premium.
Step 2. Create all the fields we need for the table.
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Figure 2. Creating all the fields in the table
Step 3. Select both RoomNumber and DateOfStay columns. Then, right-click and select Set Primary Key option in the context menu.
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R an r studio for mac os el capitan. Figure 3. Selecting two fields as primary keys
Step 4. Notice that both columns now have a key icon in the field selector area. Also, notice that the Allow Nulls property is automatically set to false for both columns.
Figure 4. Composite keys are created
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Step 5. Save the table by pressing Ctrl+S. A small dialog window will appear to allow us to enter a name for the table. Type “Reservation”, then click the OK button.
Windows server 2008 key generator. Figure 5. Saving the table
Now, we have a table named Reservation,which has two columns as composite keys. These columns will act as the primary key for the table.
Using Transact-SQL
We can also create the Reservation table we created in the previous steps using the CREATE TABLE command. Let’s follow the below steps to do that.
Step 1. Click the New Query button in the toolbar to open a new query window.
Step 2. Type the following command to create the table. Notice that we separate the primary key fields using a comma.
Step 3. Execute the SQL command to save the table into our database.
Conclusion
Creating a composite key in SQL Server is as easy as creating a primary key that consists of one column, and we can do that using SSMS or T-SQL. Jubler subtitle editor.
There might be cases when we don’t think that there is a single column that can serve as our table’s primary key. Besides, we might want to avoid adding a surrogate key (such as an integer auto-increment field) to the system. If so, we may consider using a composite key made of two or more columns.