![]() ![]() And this column will act as the foreign key of another table ITEM. Here, you can see that the column B_ID identifies all table records uniquely, so it is the primary key of the table BUYER. Let us understand composite keys with the help of the following examples: Example 1: Consider Tables Buyer and Item: Table BUYER: When over one column or field in a table are combined to achieve the task of uniquely identifying row values, then that composite key can be either a primary or a candidate key of that table. Method 1:Īfter the creation of all the columns in the table. The following examples illustrate two methods to create a primary key in SQL. A primary key can have one or as many columns as possible. In a table, there can only be one primary key. For a constraint to be recognized as a primary key, it must contain unique values throughout the row and none of the values must be NULL. The primary key is a constraint that recognizes all of the columns in a row as unique. One can say that combining a set of multiple columns in a table, results in a primary key, as it will have a unique value in every row. The composite key differs from a primary key and it is important to know how the two vary, both in syntax and usage. both columns have different data types -ĬONSTRAINT My_Composite_Key PRIMARY KEY (S_Name, Parent_Contact_No) are combined to create the composite key. columns: S_Name and Parent_Contact_No. ![]() declaring columns with different data types. SQL Queries to create the above table and declare a composite key using the columns of that table. ![]() But by combining the columns: S_Name and Parent_Contact_No, you can achieve the task of getting a unique column. In the above table, you can notice that there is no single column that has unique values. Syntax to create a composite key for a tableĬONSTRAINT COMP_NAME PRIMARY KEY (COL1, CO元, COL4)Ĭonsider an example in which you will understand the concept of composite keys in SQL using a STUDENT table. ![]() Syntax to create a composite key for a table in SQL: This scenario is one of the use cases of Composite keys. Now to overcome this issue, since you already know that the employee numbers are always unique, you can optimize your search by considering the name column along with the column of employee number as a single column. You ran into multiple employees that share the name, Rahul. But in such cases of searching by name, there is a high possibility that more than one employee shares the same name. Suppose you are handling the data of the employees of a company and you want to search for an employee named Rahul with his name in your database. Now, understand this concept with an example for better understanding. In such cases, you must combine multiple columns to create a unique key. Composite keys in SQL prove to be useful in those cases where you have a requirement of keys that can uniquely identify records for better search purposes, but you do not possess any single unique column. You already saw that the composite keys are used to identify all the rows that are involved uniquely. When Does the Composite Key Come to the Picture? The data type of the columns combined to make a composite key can be different for all the columns. Columns: The latest version of SQL supports the combining of more than 16 columns.Note that the data type of all the columns in a composite key can be different. COMPOSITE_KEY_NAME: This is the name of the new composite key created by combining two or more columns.by combining 3 columns: COL1, COL2, CO元 -ĬONSTRAINT COMPOSITE_KEY_NAME PRIMARY KEY (COL1, COL2, CO元) Syntax to declare a composite key in SQL: You can also combine all the foreign keys to create a composite key in SQL. However, the data types of different columns could differ from each other. In other words, the combination key can also be described as a primary key that is being created by using multiple columns. Even though a single column can’t identify any row uniquely, a combination of over one column can uniquely identify any record. A composite key in SQL can be defined as a combination of multiple columns, and these columns are used to identify all the rows that are involved uniquely. ![]()
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