How do I update a table based on the condition on another table in SQL?

 To update a table based on a condition from another table in SQL, you generally use a JOIN to link the two tables together and then apply the UPDATE operation. The structure of the query depends on the specific SQL database you're using, but here’s the general pattern:

Basic Syntax:

UPDATE table1
SET column1 = value
FROM table1
JOIN table2 ON table1.id = table2.id
WHERE table2.some_column = some_condition;

Explanation:

1. UPDATE table1: Specifies the table to be updated.
2. SET column1 = value: Defines the column(s) to update in table1.
3. FROM table1 JOIN table2: Joins table1 with table2 based on a common condition (e.g., matching IDs).
4. ON table1.id = table2.id: Defines the join condition (e.g., linking rows from table1 and table2 based on a common field like id).
5. WHERE table2.some_column = some_condition: Adds a condition that filters the rows from table2 and dictates which rows in table1 will be updated.

Example:

Imagine you have two tables:

• employees (with columns employee_id, name, salary)
• departments (with columns department_id, employee_id, budget)

Suppose you want to increase the salary of employees who work in a department with a budget greater than $1 million.

UPDATE employees
SET salary = salary * 1.10
FROM employees e
JOIN departments d ON e.employee_id = d.employee_id
WHERE d.budget > 1000000;

Explanation of this example:

• The UPDATE employees statement indicates that the employees table will be updated.
• SET salary = salary * 1.10 increases the salary of the selected employees by 10%.
• The FROM employees e JOIN departments d part ensures we're working with data from both the employees and departments tables.
• The WHERE d.budget > 1000000 condition ensures that only employees working in departments with a budget greater than $1 million are affected.

Note for Different Databases:

• SQL Server / PostgreSQL: The syntax shown above works well in these databases.
• MySQL: The UPDATE syntax for joining is slightly different:

UPDATE employees e
JOIN departments d ON e.employee_id = d.employee_id
SET e.salary = e.salary * 1.10
WHERE d.budget > 1000000;

• Oracle: Oracle requires a MERGE statement for updating with a join:

MERGE INTO employees e
USING departments d
ON (e.employee_id = d.employee_id)
WHEN MATCHED AND d.budget > 1000000 THEN
    UPDATE SET e.salary = e.salary * 1.10;

This is the general approach, but the exact SQL syntax might vary slightly depending on the database you're using.