Question
Answer and Explanation
In SQL, you don't use two separate WHERE clauses in a single query. Instead, you combine multiple conditions within a single WHERE clause using logical operators such as AND, OR, and NOT. Here's how you can effectively use multiple conditions:
1. Using the `AND` Operator:
- The AND operator requires that both conditions on either side of it must be true for a row to be included in the result set.
- Example: To select all customers from the 'Customers' table who are from 'London' and have an order amount greater than 100:
SELECT
FROM Customers
WHERE City = 'London' AND OrderAmount > 100;
2. Using the `OR` Operator:
- The OR operator requires that at least one of the conditions on either side of it must be true for a row to be included in the result set.
- Example: To select all customers from the 'Customers' table who are from 'London' or have an order amount greater than 100:
SELECT
FROM Customers
WHERE City = 'London' OR OrderAmount > 100;
3. Combining `AND` and `OR` Operators:
- You can combine AND and OR operators in a single WHERE clause. Use parentheses to control the order of evaluation.
- Example: To select all customers from the 'Customers' table who are from 'London' and have an order amount greater than 100, or who are from 'Paris':
SELECT
FROM Customers
WHERE (City = 'London' AND OrderAmount > 100) OR City = 'Paris';
4. Using the `NOT` Operator:
- The NOT operator negates a condition. It returns true if the condition is false, and vice versa.
- Example: To select all customers from the 'Customers' table who are not from 'London':
SELECT
FROM Customers
WHERE NOT City = 'London';
Key Points:
- Always use logical operators (AND, OR, NOT) to combine multiple conditions within a single WHERE clause.
- Use parentheses to clarify the order of operations when combining AND and OR operators.
- Avoid using multiple WHERE clauses in a single SQL query; it's not the correct syntax.
By understanding how to use these logical operators, you can create complex and precise queries to retrieve the exact data you need from your database.