Introduction: When working with SQL Server, you may often need to update one table’s values using data from another table. This can be achieved by combining the UPDATE statement with a SELECT query. In this article, we will explain how to update from a SELECT using both JOIN and subqueries. We’ll also provide practical examples to help you apply these techniques.

What is the UPDATE FROM SELECT in SQL Server?

The UPDATE statement is used to modify existing records in a table. When combined with a SELECT statement, it allows you to update data based on the results retrieved from one or more tables. This is particularly useful for updating values using data from a related table or complex conditions.

How to Use UPDATE with JOIN in SQL Server

One of the most common ways to update data from a SELECT is by using a JOIN between the target table and the source table. The JOIN allows you to fetch data from another table and use it to update the target table.

Syntax for UPDATE from SELECT with JOIN:

sqlCopy codeUPDATE target_table
SET target_table.column_name = source_table.column_name
FROM target_table
INNER JOIN source_table ON target_table.id = source_table.id
WHERE condition;

Example: Update Employee Salaries Based on Department’s Average Salary

Suppose you have two tables: employees and departments. You want to update the salaries of employees to match their department’s average salary:

sqlCopy codeUPDATE e
SET e.salary = d.average_salary
FROM employees e
INNER JOIN departments d ON e.department_id = d.department_id
WHERE e.salary < d.average_salary;

In this query:

• employees is the target table.
• departments is the source table.
• The INNER JOIN ensures that only employees within a department are updated based on the department’s average salary.

Using a Subquery in UPDATE from SELECT

Another approach to update data is using a subquery in the SET clause. Subqueries allow you to retrieve a single value from another table based on the current record in the target table.

Syntax for UPDATE using Subquery:

sqlCopy codeUPDATE target_table
SET column_name = (SELECT value FROM source_table WHERE condition)
WHERE condition;

Example: Update Employee Salaries Using a Subquery

Here’s a simple example using a subquery:

sqlCopy codeUPDATE employees
SET salary = (SELECT average_salary FROM departments WHERE departments.department_id = employees.department_id)
WHERE salary < (SELECT average_salary FROM departments WHERE departments.department_id = employees.department_id);

In this case:

• The SET clause uses a subquery to fetch the average_salary from the departments table for each employee.
• The WHERE clause ensures that only employees earning less than their department’s average salary are updated.

Best Practices for UPDATE with SELECT

1. Use Joins for Better Performance: When updating from multiple tables, using a JOIN is generally more efficient than using subqueries, especially for large datasets.
2. Check for Data Integrity: Always ensure your join conditions or subqueries are correct to avoid updating the wrong records.
3. Limit Updates: Use the WHERE clause effectively to limit which rows are updated. This helps prevent unintended data modifications.

Conclusion:

Updating data in SQL Server from a SELECT query is a powerful way to manipulate and modify tables based on data from one or more sources. By using INNER JOINs or subqueries, you can efficiently update values across multiple tables. Always ensure your conditions and joins are correct to maintain data integrity.

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