In today’s lesson you’re going to learn how to query results using the WHERE clause.
The objectives of today’s lesson are to:
- Learn to use more than one compare condition at a time using Boolean logic
- Comprehensive example with Select, Where, and Order By
Using Multiple Conditions to Query Results
In previous lesson’s we learned how the where clause is used filter out any records where the where condition is FALSE. Did you also know you can also string conditions together to create more complex conditions? To do so we can use the AND, OR, and NOT operators.
These three operators are used for the most common aspects of Boolean logic. Regardless of which operator is used the result always boils down to one of two outcomes: TRUE or FALSE.
Where clauses become really interesting when we consider combining more than one field to filter a result. For instance, using our sample database as an example, we may want to find large orders, such as those with a quantity greater than 10 and price greater than $5.00. This could be written as
SELECT OrderID, ProductID, UnitPrice, Quantity FROM OrderDetails WHERE UnitPrice > 5.00 AND Quantity > 10;
Records are only included when both conditions are TRUE.
Now that we know how to write more complicated conditions, let’s learn more about the various Boolean operators. We’ll start with AND.
Boolean AND Operator
The AND operator returns a TRUE only if all conditions are also TRUE. The following truth table shows all combinations of values the condition (A AND B).
|Condition A||Condition B||Result|
In SQL we can string a where clause together using to test multiple fields. For instance if you’re looking for customers from Midland, TX you could write
WHERE State = 'TX' AND City = 'Midland'
You can also use the AND operator to create range condition, much like we do with BETWEEN.
Using our previous example of wanting to find TotalPrices that fall within $10. 00 and $20.00 dollars we would write
SELECT OrderID, ProductID, UnitPrice * Quantity AS TotalPrice FROM OrderDetails WHERE TotalPrice >= 10.00 AND TotalPrice <= 20.00;
This returns the same result as
SELECT OrderID, ProductID, UnitPrice * Quantity AS TotalPrice FROM OrderDetails WHERE TotalPrice BETWEEN 10.00 AND 20.00;
Tip! Keep in mind that Boolean logic AND doesn’t completely translate to English “and.” For instance If were to say Search for all customers in the states of Florida and Georgia, you would most likely know I meant find all customer from either Florida or Georgia. You wouldn’t think I meant for you to find all customers that are in both Florida and Georgia.
Boolean OR Operator
The OR operator returns a TRUE when one or more conditions are also TRUE. Here is the Truth table for the OR operator. You’ll see that in every case one of the conditions is true, so is the end result.
|Condition A||Condition B||Result|
The where clause to select all Customers from either Texas or Florida is
WHERE State = 'FL' OR State = 'TX'
Multiple OR clauses can be connected together to behave similar to the IN statement. In this manner they act as a membership condition.
To find all customer that lead their company we would write
SELECT CompanyName, ContactName, ContactTitle FROM Customers WHERE ContactTitle = 'CEO' OR ContactTitle = 'Owner' OR ContactTitle = 'President';
This is the same as this
SELECT CompanyName, ContactName, ContactTitle FROM Customers WHERE ContactTitle IN ('CEO', 'Owner', 'President');
Boolean NOT Operator
The not operator takes a condition and changes it to the opposite. So given TRUE, the NOT operator changes it to FALSE. Some examples of expressions using the NOT statement include:
- NOT IN (‘TX’, ‘FL’) – Accept every state except Texas and Florida
- NOT IN (‘CEO’, ‘Owner’, ‘President’) – Accept everyone that isn’t an owner.
The NOT statement can also be used in combination with AND and OR. However, to explain this, we first need to understand which order the conditions are evaluated and how to group them together.
In other words, we need to learn about parenthesis and used them much in the same way you would use them with adding and multiplying numbers.
Combining Boolean Operators
The order Boolean operators are executed is important and isn’t arbitrary. Much like in arithmetic, where multiplication occurs before additions, in Boolean operators, AND is evaluated before OR.
Can you tell what’s wrong in this photo? What did they really mean?
In English they are trying to say that you can have your choice of soup with either a spring roll or crab Rangoon, but since the AND condition is evaluated first, the SQL engine sees these choices:
- Hot & Sour soup
- Wonton Soup
- Egg Drop Soup and Spring Roll
- Crab Rangoon
You can also use parenthesis. The expression inside of the parenthesis is evaluated first.
Let’s say you wanted to return all customers who are not owners. How could we do this?
ContactTitle = 'CEO' OR ContactTitle = 'Owner' OR ContactTitle = 'President'
Now, to get those that aren’t owners we need to reverse the logic as
NOT (ContactTitle = 'CEO' OR ContactTitle = 'Owner' OR ContactTitle = 'President')
Notice the use of parenthesis, the condition within the parenthesis are evaluated first, then the NOT condition second.
Suppose we need to find all large OrdersDetails. If we consider a large Order to be one where the Quantity > 100 or the UnitPrice > 10 and we want to order them by the total price. How would we go about this? Lets try
SELECT OrderDetailID, UnitPrice, Quantity ,UnitPrice*Quantity as TotalPrice FROM OrderDetails WHERE UnitPrice > 10.00 OR Quantity > 100 ORDER BY TotalPrice;
Now we could refine this further by then asserting that the TotalPrice is greater than 1000.
The modified query is
SELECT OrderDetailID, UnitPrice, Quantity ,UnitPrice*Quantity as TotalPrice FROM OrderDetails WHERE (UnitPrice > 10.00 OR Quantity > 100) AND TotalPrice > 1000.00 ORDER BY TotalPrice;
I added the parenthesis to for the or statements to be evaluated before the AND; otherwise the statement would have a different result.
It’s important to practice! Use the sample database to answer these questions.
- Find Customers from the State of Illinois or Nevada
- Select OrderID, OrderDate, and FreightCharge for all Orders whose freight charges are within the $4.00 to $7.00 range — don’t use the BETWEEN operator
- Find all OrderDetails whose TotalPrice is greater than 100.00 but less than 1000.00
Answers to Exercises
Correction: In problem number two, I use the OR clause when comparing the FreightCharge, I should have used AND.
SELECT OrderID, OrderDate, FreightCharge FROM Orders WHERE FreightCharge >= 4.00 AND FreightCharge <= 7.00
Congratulations! You just learned how to use multiple conditions to create more sophisticated filtering conditions. More tutorials are to follow! Remember! I want to remind you all that if you have other questions you want answered, then post a comment or tweet me. I’m here to help you. What other topics would you like to know more about?