Get Item By Composite Primary Key Mysql With Code Examples
With this article, we’ll look at some examples of how to address the Get Item By Composite Primary Key Mysql problem .
SELECT * FROM bb_bulletin WHERE (OfficeCode, IssuerId, BulletinDtm) = (20001, 1, '2011-01-07 14:04:40');The various approaches to solving the Get Item By Composite Primary Key Mysql problem are summarised in the following code.
SELECT * FROM games WHERE (platformid, genreid) = (1, 10)With many examples, we have shown how to resolve the Get Item By Composite Primary Key Mysql problem.
How do I find the composite key in MySQL?
You can use aggregate function count(*). If it returns a value greater than 1, that would mean the table has composite primary key.05-Nov-2019
How do you get a composite key?
A composite key is made by the combination of two or more columns in a table that can be used to uniquely identify each row in the table when the columns are combined uniqueness of a row is guaranteed, but when it is taken individually it does not guarantee uniqueness, or it can also be understood as a primary key made 26-Apr-2021
A table can have only one primary key, which may consist of single or multiple fields. When multiple fields are used as a primary key, they are called a composite key. If a table has a primary key defined on any field(s), then you cannot have two records having the same value of that field(s).
How do I create a composite primary key in MySQL?
Now, you can execute the ALTER TABLE statement to add a MySQL Composite Primary key as follows: mysql> alter table Orders ADD PRIMARY KEY (order_id, product_id);21-Jan-2022
What is composite search key?
A composite key, in the context of relational databases, is a combination of two or more columns in a table that can be used to uniquely identify each row in the table. Uniqueness is only guaranteed when the columns are combined; when taken individually the columns do not guarantee uniqueness.15-Jun-2017
Can foreign key reference composite key?
The foreign key can not refer to part of composite primary key of other table. Because it is supposed to be one-to-one relationship and if you refer just part of primary-key, there might be more than one parent record available, which is neither allowed nor possible.11-Jun-2020
How do I select a composite key in SQL?
You will need to list all columns that are contained within the composite primary key in the predicate. A potential solution is to build a table that stores all primary key combinations for the fact table. You can add an identity field to that table and make it a foreign key constraint on your table.04-Apr-2018
How does composite key work in SQL?
A composite key is a combination of two or more columns in a table that can be used to uniquely identify each row in the table when the columns are combined uniqueness is guaranteed, but when it taken individually it does not guarantee uniqueness.
Is composite key and super key same?
A super key uniquely identifies a row. It could be made up of one column or many. A composite key is a key made of more than one column. If a Super Key is made of more than one column it is also a composite.06-Jun-2014
Can you have composite primary key SQL?
Composite key, or composite primary key, refers to cases where more than one column is used to specify the primary key of a table. In such cases, all foreign keys will also need to include all the columns in the composite key.27-Sept-2016
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. Even though a single column can’t identify any row uniquely, a combination of over one column can uniquely identify any record. In other words, the combination key can also be described as a
primary key that is being created by using multiple columns. However, the data types of different columns could differ from each other. You can also combine all the foreign keys to create a composite key in SQL. Syntax to
declare a composite key in SQL: ------ Syntax to create a composite key ------ ------ by combining 3 columns: COL1, COL2, COL3 ----- CONSTRAINT COMPOSITE_KEY_NAME PRIMARY KEY (COL1, COL2, COL3) Description: Note that the data type of all the columns in a composite key can be different. You already saw that the composite keys are used to identify all the rows that are involved uniquely. 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. In such cases, you must combine multiple columns to create a unique key. Now, understand this concept with an example for better understanding. 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. But in such cases of searching by name, there is a high possibility that more than one employee shares the same name. This
happened in this case, too. You ran into multiple employees that share the name, Rahul. 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. This scenario is one of the use cases of Composite keys. Syntax to create a composite key for a
table in SQL: ---- Syntax to create a composite key for a table ---- by combining some columns Create table table_name ( COL1 data_type_1 NOT NULL, COL2 data_type_2 NOT NULL, COL3 data_type_3, COL4 data_type_4, COLN data_type_n, ----- Declare the composite key ----- ---- here COL1, COL3, and COL4
are ---- ---- forming up the composite key ---- CONSTRAINT COMP_NAME PRIMARY KEY (COL1, COL3, COL4) ); Consider an example in which you will understand the concept of composite keys in SQL using a STUDENT table. S_Name S_Class Parent_Contact_No S_Age Mehul 6 8700867330 11 Rashi 5 8700867330 10 Mehul 6 9990155289 11 Vansh 7 9354226009 12 Ishita 9 8681012213 14 In the above table, you can notice that there is no single column that has unique values. But by combining the columns: S_Name and Parent_Contact_No, you can achieve the task of getting a unique column. SQL Queries to create the above table and declare a composite key using the columns of that table. ---- create a table named STUDENT
---- Create table STUDENT ( ---- declaring columns with different data types ---- S_Name varchar(30), S_Class integer, Parent_Contact_No integer, S_Age integer, ---- declare the composite key ---- ---- columns: S_Name and Parent_Contact_No ---- ---- are combined to create the composite key ---- ---- both columns have different data types ---- CONSTRAINT
My_Composite_Key PRIMARY KEY (S_Name, Parent_Contact_No) ); ---- add values to the table ---- Insert into STUDENT values('Mehul', 6, 8700867330, 11); Insert into STUDENT values('Rashi', 5, 8700867330, 10); Insert into STUDENT values('Mehul', 6, 9990155289, 11); Insert into STUDENT values('Vansh', 7, 9354226009, 12); Insert into STUDENT values('Ishita', 9, 8681012213, 14); ---- select all values of the table ---- ---- to display the
content ---- Select * from STUDENT The composite key differs from a primary key and it is important to know how the two vary, both in syntax and usage. 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 primary key is a constraint that recognizes all of the columns in a row as unique. 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. In a table, there can only be one primary key. A primary key can have one or as many columns as possible. Syntax to create a primary key in SQL: ---- create a table ---- Create table Table_Name ( COL1 data_type1 NOT NULL, COL2 data_type2 NOT NULL, COL3 data_type3 ----
declare the primary key ---- ---- after declaring all columns ---- PRIMARY KEY (COL1)What Is a Composite Key in SQL?
When Does the Composite Key Come to the Picture?
Table STUDENT:
How to Use the Composite Key in SQL?
Primary Key Declaration
At the Column Level:
);
At the Row-Level:
---- create a table ----
Create table Table_Name
(
---- declare the primary key ----
---- while declaring the columns ----
COL1 data_type1 PRIMARY KEY,
COL2 data_type2 NOT NULL,
COL3 data_type3,
);
The following examples illustrate two methods to create a primary key in SQL.
Method 1:
After the creation of all the columns in the table.
---- create a table named STUDENT ----
Create table STUDENT
(
---- declaring columns with different data types ----
S_ID integer NOT NULL,
S_Name varchar(30) NOT NULL,
S_Class integer,
Parent_Contact_No integer,
S_Age integer,
---- declare the primary key ----
PRIMARY KEY (S_ID)
);
Method 2:
While creating a column in the table.
---- create a table named STUDENT ----
Create table STUDENT
(
---- declare the primary key -----
------ while creating the column ----
S_ID integer PRIMARY KEY,
S_Name varchar(30),
S_Class integer NOT NULL,
Parent_Contact_No integer,
S_Age integer,
);
Composite Key Declaration
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.
Syntax to create a composite key in SQL:
---- create a table ----
Create table table_name (
COL1 data_type_1 NOT NULL,
COL2 data_type_2 NOT NULL,
COL3 data_type_3,
COLN data_type_n,
----- Declare the composite key -----
CONSTRAINT COMP_NAME PRIMARY KEY (COL1, COL3, COL4)
);
Examples of Composite Keys in SQL.
Let us understand composite keys with the help of the following examples:
Example 1: Consider Tables Buyer and Item:
Table BUYER:
B_ID (Primary Key) | B_Name | B_Contact_No | B_Address |
101 | Mohan | 8700867330 | Delhi |
102 | Pankaj | 8681012213 | Noida |
103 | Diya | 8090100224 | Saket |
104 | Rohit | 9354226009 | Gurugram |
105 | Gaurav | 9990155289 | Noida |
Here, you can see that the column B_ID identifies all table records uniquely, so it is the primary key of the table BUYER. And this column will act as the foreign key of another table ITEM.
SQL queries for the table BUYER.
Create table BUYER
(
---- declaring columns of the table ----
B_ID int NOT NULL,
B_Name varchar (30),
B_Contact_No int,
B_Address varchar (50),
---- declare the primary key ----
PRIMARY KEY (B_ID)
);
---- add values to the table ----
Insert into BUYER values(101, 'Mohan', 8700867330, 'Delhi');
Insert into BUYER values(102, 'Pankaj', 8681012213, 'Noida');
Insert into BUYER values(103, 'Diya', 8090100224, 'Saket');
Insert into BUYER values(104, 'Rohit', 8090100224, 'Gurugram');
Insert into BUYER values(105, 'Gaurav', 9990155289, 'Noida');
---- select all values of the table ----
---- to display the content ----
Select * from BUYER
Table ITEM:
B_ID (Foreign Key) | Order_No | I_ID | I_Name | I_Quantity |
101 | 1 | 201 | Laptop | 1 |
102 | 2 | 205 | Headphones | 2 |
103 | 3 | 202 | Mobile | 2 |
102 | 2 | 202 | Keyboard | 1 |
101 | 1 | 204 | Speaker | 4 |
B_ID, Order_No, I_ID are combined to form the composite key.
In the table ITEM, the column B_ID acts as a foreign key, and there is no column that can act as the primary key. This means that there is no field that can be used to identify all the table records uniquely. So to make a primary key, you can create a composite key by combining the columns: B_ID, Order_No, and I_ID. Together combined, these 3 columns can become a primary key for the table ITEM.3
SQL queries for the table ITEM.
---- create a table named ITEM ----
Create table ITEM
(
---- declaring columns of the table ----
B_ID int NOT NULL,
Order_No int,
I_ID int(10),
I_Name varchar (20),
I_Quantity int,
---- declare the composite key ----
CONSTRAINT MyCompositeKey PRIMARY KEY (B_ID, Order_No, I_ID)
);
---- add values to the table ----
Insert into ITEM values(101, 1, 201, 'Laptop', 1);
Insert into ITEM values(102, 2, 205, 'HeadPhones', 2);
Insert into ITEM values(103, 3, 202, 'Mobile', 2);
Insert into ITEM values(102, 2, 202, 'Keyboard', 1);
Insert into ITEM values(101, 1, 204, 'Speaker', 4);
---- select all values of the table ----
---- to display the content ----
Select * from ITEM
Example 2: Consider the Table Student:
Table STUDENT:
S_Name | S_Class | Parent_Contact_No | Payment_Status |
Mehul | 6 | 8700867330 | Yes |
Rashi | 5 | 8700867330 | Yes |
Mehul | 6 | 9990155289 | No |
Vansh | 7 | 9354226009 | Yes |
Ishita | 9 | 8681012213 | No |
S_Name, S_Class, Parent_Contact_No are combined to form the composite key.
The table depicted above represents the records of the students with their fee payment status. Here, also you can notice that no individual column can be used to identify all values of the table as unique. However, you can combine the first three columns: S_Name, S_Class, and Parent_Contact_No to create a composite key.
SQL queries for the table ITEM.
---- create a table named STUDENT ----
Create table STUDENT
(
---- declaring columns with different data types ----
S_Name varchar (30),
S_Class integer,
Parent_Contact_No integer,
Payment_Status varchar (4),
---- declare the composite key ----
---- columns: S_Name, S_Class, and Parent_Contact_No ----
---- are combined to create the composite key ----
---- all three columns have different data types ----
CONSTRAINT My_Composite_Key PRIMARY KEY (S_Name, S_Class, Parent_Contact_No)
);
---- add values to the table ----
Insert into STUDENT values('Mehul', 6, 8700867330, 'Yes');
Insert into STUDENT values('Rashi', 5, 8700867330, 'Yes');
Insert into STUDENT values('Mehul', 6, 9990155289, 'No');
Insert into STUDENT values('Vansh', 7, 9354226009, 'Yes');
Insert into STUDENT values('Ishita', 9, 8681012213, 'No');
---- select all values of the table ----
---- to display the content ----
Select * from STUDENT
How to Alter and Drop Composite Keys?
Like any other column in a table, composite keys can also be operated with the alter commands. You can easily perform add or drop operations in a table by using the alter command with the composite key in SQL. To add additional columns in the set of columns forming the composite key, you can use the alter-add command. And to delete a column from the set of columns combined together to form the composite key, you can use the alter-drop command.
- ALTER-ADD command: To add additional columns to the existing set of columns forming the composite key.
Syntax
---- syntax to add columns to an ----
---- existing composite key ----
ALTER table table_name
ADD CONSTRAINT Constraint_name PRIMARY KEY (COL1, COL2, COLN);
---- this will add COL1, COL2, and COLN ----
---- to the composite key of the table ----
Example
---- syntax to add columns to an ----
---- existing composite key ----
---- of the table STUDENT ----
ALTER table STUDENT
ADD CONSTRAINT MyCompKey_Scores PRIMARY KEY ( Scores, Percentage );
---- this will add columns named ----
---- Scores and Percentage ----
---- to the composite key of the table ----
- ALTER-DROP command: To delete a column from the existing set of columns forming the composite key.
Syntax
---- syntax to delete a column from an ----
---- existing composite key ----
ALTER table table_name
DROP CONSTRAINT Constraint_Name;
---- this will delete the constraint ----
---- from the composite key of the table ----
Example
---- syntax to delete a column from an ----
---- existing composite key ----
---- of the table STUDENT ----
ALTER table STUDENT
DROP CONSTRAINT MyCompKey_Scores;
---- this will delete the constraint ----
---- from the composite key of the table ----
How to Create Composite Keys in Mysql, SQL Server, and Postgresql?
MySQL
Syntax
---- Syntax to create a composite key for a table
---- by combining some columns
Create table table_name (
COL1 data_type_1,
COL2 data_type_2,
COL3 data_type_3,
COLN data_type_n,
----- Declare the composite key -----
---- here COL1 and COL3 are ----
---- forming up the composite key ----
PRIMARY KEY (COL1, COL)
);
Example
Create table STUDENT
(
---- declaring columns with different data types ----
S_ID integer,
S_Class integer,
S_Age integer,
S_Phone integer,
---- declare the primary key ----
PRIMARY KEY (S_ID, S_Phone)
);
SQL Server
Syntax
---- Syntax to create a composite key for a table
---- by combining some columns
Create table table_name (
COL1 data_type_1,
COL2 data_type_2,
COL3 data_type_3,
COLN data_type_n,
----- Declare the composite key -----
---- here COL1 and COL3 are ----
---- forming up the composite key ----
PRIMARY KEY (COL1, COL)
);
Example
Create table STUDENT
(
---- declaring columns with different data types ----
S_ID integer,
S_Class integer,
S_Age integer,
S_Phone integer,
---- declare the primary key ----
PRIMARY KEY (S_ID, S_Phone)
);
Postgresql
Syntax
---- Syntax to create a composite key for a table
---- by combining some columns
Create table table_name (
COL1 data_type_1,
COL2 data_type_2,
COL3 data_type_3,
COLN data_type_n,
----- Declare the composite key -----
---- here COL1 and COL3 are ----
---- forming up the composite key ----
PRIMARY KEY (COL1, COL)
);
Example
Create table STUDENT
(
---- declaring columns with different data types ----
S_ID integer,
S_Class integer,
S_Age integer,
S_Phone integer,
---- declare the primary key ----
PRIMARY KEY (S_ID, S_Phone)
);
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Wrapping Up!
In this article, you have learned about the composite key in SQL. This article looked into the basics of composite keys in SQL and how to use them. You dived deep into the primary key and composite keys, their usage, and examples. It also discussed examples of how to create composite keys in MySQL, SQL Server, and Postgresql.
One of the best ways to learn the concepts of SQL in a detailed and hands-on manner is to take up the SQL Certification Training Course provided by Simplilearn. With the help of this training course, you will be able to master SQL and improve your career aspects. This course covers basics to advanced topics such as Database, relationships, query tools, SQL commands, aggregate functions, transaction controls, views, procedures, etc. Mastering these topics will open doors to lucrative careers such as a Business Analyst, Data Analyst, etc.
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