Join is an operation used in SQL to join two or more tables.
In other words, a join queries two or more tables at the same time to create a result table. This provides a more detailed and meaningful use of the data.
The join operation is performed by comparing specific columns from both tables. As a result of this comparison, the two tables are joined over the specified columns. The join operation displays the joined tables as a new virtual table.
Join is an operation in the SQL language used to combine two or more tables. In other words, the Join operation serves to query two or more tables simultaneously to create a result table. This process allows data to be used in a more detailed and meaningful way.
The Join operation is performed by comparing certain columns from both tables. As a result of this comparison, the two tables are joined based on the specified columns. The Join operation displays the combined tables as a new virtual table.
There are different types of joins in SQL. These are:
The most frequently used type of join. It performs the joining operation based on common values between two tables and retrieves only records with common values. During the operation, you can write just JOIN instead of INNER JOIN in the query. The result table can be further customized using the WHERE clause. When using JOIN, it's important not to use "null" values as a merge condition. Two different representations can be used in table merging.
SELECT * FROM tablo1 INNER JOIN tablo2 ON tablo1.alan1 karşılaştırma tablo2.alan2
SELECT * FROM tablo1, table2 WHERE table1.field = table2.field
Cross Join in SQL is a type of join that combines each row of one table with every row of another table. This operation is also known as Cartesian product.
The Cross Join operation matches every row from one table with every row from another table, without any condition or column comparison. Therefore, when performed on large data sets, the Cross Join can lead to performance issues.
Cross join is used when there is no relationship or condition between the two tables to be joined. For instance, consider a customer table and a product table. By merging each row of these tables with every row of the other, a virtual table can be created showing all products that all customers could potentially purchase.
Cross join is typically not used with other join types and is chosen only in special cases. However, it can be useful in some data analysis or processing scenarios. Below are SQL commands written in two different representations for the department and employee tables:
SELECT * FROM employee CROSS JOIN departmentSELECT * FROM employee, department
Outer join in SQL is a join operation that combines two tables. With outer join, records don't need to match in both tables. Compared to inner join, outer join doesn't repeat common records and also retrieves records that exist in one table but not in the other. It has three different variations:
The Left outer join displays all records from the left table and the matching records from the right table without repetition. If there is no matching record in the right table, it shows a null value.
For instance, consider a customer table and an order table. To show whether each customer in the customer table has placed an order or not, a left outer join can be used. This way, all customer records are retrieved and combined with orders. For customers without an order, null values are displayed.
Another example, joining the employee and department tables using left outer join:
SELECT * FROM employee LEFT OUTER JOIN department ON employee.DepartmentID = department.DepartmentID;
The Right outer join displays all records from the right table and the matching records from the left table without repetition. If there's no matching record in the left table, it displays a null value.
For example, using a customer table and an order table, to find out if every order in the order table belongs to a customer, a right outer join can be used. In this case, all orders are retrieved and combined with the customer table. For orders without a customer, null values are shown.
Another example of joining the employee and department tables using a right outer join:
SELECT * FROM employee RIGHT OUTER JOIN department ON employee.DepartmentID = department.DepartmentID
Full outer join displays all records from both tables, without repeating common records. If there's no matching record in the right table, it shows a null value for the left table and vice versa.
For instance, with a customer table and an order table, to prepare a report displaying all customers and all orders, a full outer join can be used. This way, all customer records are retrieved and combined with the order table. Null values are not displayed for matching records in both the customer and order tables.
Another example of joining the employee and department tables using a full outer join:
SELECT * FROM employee FULL OUTER JOIN department ON employee.DepartmentID = department.DepartmentID
Self join in SQL is an operation that combines a table with itself. In this operation, the goal is to match a column from a table with another column from the same table and retrieve data from that table.
For instance, consider an employee table where each employee has a manager recorded. In this case, a joining operation can be performed to match employees with their managers. This operation involves creating a copy of the employee table and joining it with the manager column to display the name and other details of each employee's manager.
Self join is often used to model hierarchical structures. For example, in a category table, categories might have sub-categories. In such cases, a self join can be used to combine sub-categories with their main categories.
For instance, if there's an employees table containing information about employees and their managers. In this case, to retrieve a list of employees and their managers, the following SQL statement can be used:
SELECT e.employee_name, m.employee_name AS manager_nameFROM employees eINNER JOIN employees m ON e.manager_id = m.employee_id
In this query, the employees table is used twice. In its first use, the table named "e" contains information about employees, while in its second use, the table named "m" contains information about managers.
In our next learning content, we will closely examine the concept of subqueries.
Join is an operation used in SQL to join two or more tables.
In other words, a join queries two or more tables at the same time to create a result table. This provides a more detailed and meaningful use of the data.
The join operation is performed by comparing specific columns from both tables. As a result of this comparison, the two tables are joined over the specified columns. The join operation displays the joined tables as a new virtual table.
Join is an operation in the SQL language used to combine two or more tables. In other words, the Join operation serves to query two or more tables simultaneously to create a result table. This process allows data to be used in a more detailed and meaningful way.
The Join operation is performed by comparing certain columns from both tables. As a result of this comparison, the two tables are joined based on the specified columns. The Join operation displays the combined tables as a new virtual table.
There are different types of joins in SQL. These are:
The most frequently used type of join. It performs the joining operation based on common values between two tables and retrieves only records with common values. During the operation, you can write just JOIN instead of INNER JOIN in the query. The result table can be further customized using the WHERE clause. When using JOIN, it's important not to use "null" values as a merge condition. Two different representations can be used in table merging.
SELECT * FROM tablo1 INNER JOIN tablo2 ON tablo1.alan1 karşılaştırma tablo2.alan2
SELECT * FROM tablo1, table2 WHERE table1.field = table2.field
Cross Join in SQL is a type of join that combines each row of one table with every row of another table. This operation is also known as Cartesian product.
The Cross Join operation matches every row from one table with every row from another table, without any condition or column comparison. Therefore, when performed on large data sets, the Cross Join can lead to performance issues.
Cross join is used when there is no relationship or condition between the two tables to be joined. For instance, consider a customer table and a product table. By merging each row of these tables with every row of the other, a virtual table can be created showing all products that all customers could potentially purchase.
Cross join is typically not used with other join types and is chosen only in special cases. However, it can be useful in some data analysis or processing scenarios. Below are SQL commands written in two different representations for the department and employee tables:
SELECT * FROM employee CROSS JOIN departmentSELECT * FROM employee, department
Outer join in SQL is a join operation that combines two tables. With outer join, records don't need to match in both tables. Compared to inner join, outer join doesn't repeat common records and also retrieves records that exist in one table but not in the other. It has three different variations:
The Left outer join displays all records from the left table and the matching records from the right table without repetition. If there is no matching record in the right table, it shows a null value.
For instance, consider a customer table and an order table. To show whether each customer in the customer table has placed an order or not, a left outer join can be used. This way, all customer records are retrieved and combined with orders. For customers without an order, null values are displayed.
Another example, joining the employee and department tables using left outer join:
SELECT * FROM employee LEFT OUTER JOIN department ON employee.DepartmentID = department.DepartmentID;
The Right outer join displays all records from the right table and the matching records from the left table without repetition. If there's no matching record in the left table, it displays a null value.
For example, using a customer table and an order table, to find out if every order in the order table belongs to a customer, a right outer join can be used. In this case, all orders are retrieved and combined with the customer table. For orders without a customer, null values are shown.
Another example of joining the employee and department tables using a right outer join:
SELECT * FROM employee RIGHT OUTER JOIN department ON employee.DepartmentID = department.DepartmentID
Full outer join displays all records from both tables, without repeating common records. If there's no matching record in the right table, it shows a null value for the left table and vice versa.
For instance, with a customer table and an order table, to prepare a report displaying all customers and all orders, a full outer join can be used. This way, all customer records are retrieved and combined with the order table. Null values are not displayed for matching records in both the customer and order tables.
Another example of joining the employee and department tables using a full outer join:
SELECT * FROM employee FULL OUTER JOIN department ON employee.DepartmentID = department.DepartmentID
Self join in SQL is an operation that combines a table with itself. In this operation, the goal is to match a column from a table with another column from the same table and retrieve data from that table.
For instance, consider an employee table where each employee has a manager recorded. In this case, a joining operation can be performed to match employees with their managers. This operation involves creating a copy of the employee table and joining it with the manager column to display the name and other details of each employee's manager.
Self join is often used to model hierarchical structures. For example, in a category table, categories might have sub-categories. In such cases, a self join can be used to combine sub-categories with their main categories.
For instance, if there's an employees table containing information about employees and their managers. In this case, to retrieve a list of employees and their managers, the following SQL statement can be used:
SELECT e.employee_name, m.employee_name AS manager_nameFROM employees eINNER JOIN employees m ON e.manager_id = m.employee_id
In this query, the employees table is used twice. In its first use, the table named "e" contains information about employees, while in its second use, the table named "m" contains information about managers.
In our next learning content, we will closely examine the concept of subqueries.
