How to Specifies which columns and values that should be updated in a table in SQL ?

To specify which columns and values should be updated in a table in SQL, you can use the UPDATE statement with the SET clause. There are multiple methods to do it:

Method 1: Update a single column

UPDATE table_name
SET column_name = new_value
WHERE condition;

Example:

UPDATE customers
SET city = 'New York'
WHERE customer_id = 1;

This will update the city column of the customers table to New York where the customer_id is 1.

Method 2: Update multiple columns

UPDATE table_name
SET column1 = new_value1, column2 = new_value2, ...
WHERE condition;

Example:

UPDATE customers
SET city = 'New York', country = 'USA'
WHERE customer_id = 1;

This will update the city and country columns of the customers table to New York and USA respectively where the customer_id is 1.

Method 3: Update with subquery

UPDATE table_name
SET column_name = (SELECT expression FROM table_name WHERE condition)
WHERE condition;

Example:

UPDATE orders
SET total_amount = (SELECT SUM(price*quantity) FROM order_items WHERE order_id = orders.order_id)
WHERE order_id = 1;

This will update the total_amount column of the orders table with the sum of price*quantity from the order_items table where the order_id is 1.

Note: It is important to include the WHERE clause in the UPDATE statement to avoid updating all the rows in the table.