Combining logical operators
You can combine the AND, OR, and NOT operators in various ways to create more sophisticated filters for your data. However, you need to be careful about the order of precedence and the use of parentheses to avoid ambiguity and errors. The order of precedence for logical operators in SQL is:
This means that the NOT operator is evaluated first, then the AND operator, and finally the OR operator. For example, the following query will select customers who live in New York or Los Angeles and have spent more than $1000 in the last year:
SELECT name, city, total_spent
FROM customers
WHERE (city = 'New York' OR city = 'Los Angeles') AND total_spent > 1000;
The parentheses are used to group the OR condition and ensure that it is evaluated before the AND condition. Without the parentheses, the query would be interpreted as:
SELECT name, city, total_spent
FROM customers
WHERE city = 'New York' OR (city = 'Los Angeles' AND total_spent > 1000);
This would return customers who live in New York regardless of their spending, and customers who live in Los Angeles and have spent more than $1000. This is not the same as the original query, and may not be what you intended.
Therefore, it is advisable to use parentheses whenever you combine multiple logical operators in your queries, to make your logic clear and avoid mistakes.