Skip to content

Filtering 🚦

Success

The WHERE clause is used to filter the rows in a query. It's like the "filter" feature in Excel!

Note

The WHERE clause is optional. If you use it, it must come after the FROM clause.

WHERE is how we filter rows

So far, we've been able to "open a file" by using SELECT and FROM.

To filter rows, use the WHERE clause and specify the condition that you want to filter by.

Excel is convenient and gives us a pop-up box to select the values we want to filter by, but for SQL we have to write the condition ourselves. The way that we write the condition is similar to how we write the conditions for the IF function in Excel using the following operators:

  • < (less than)
  • <= (less than or equal to)
  • > (greater than)
  • >= (greater than or equal to)
  • = (equals)
  • <> (does not equal), also written as !=

For example, we could filter the HumanResources.Department table for people who have the first name Rob using:

SELECT
    DepartmentID,
    Name,
    GroupName
FROM HumanResources.Department
WHERE DepartmentID = 5
;
DepartmentID Name GroupName
5 Purchasing Inventory Management

Tip

The WHERE clause "sounds like" English, so the query above can be read as:

"Select the department ID, name, and group name from the HumanResources.Department table where the department ID is 5".

Similarly, we could filter the HumanResources.Department table for departments whose ID is less than or equal to 5 using:

SELECT
    DepartmentID,
    Name,
    GroupName
FROM HumanResources.Department
WHERE DepartmentID <= 5
;
DepartmentID Name GroupName
1 Engineering Research and Development
2 Tool Design Research and Development
3 Sales Sales and Marketing
4 Marketing Sales and Marketing
5 Purchasing Inventory Management

Conditions can be combined with AND and OR

If you wanted to have multiple conditions in an IF statement in Excel, you'd need to use the AND or OR functions:

=IF(AND(A1 = "Alan", B1 <= 5), "Yes", "No")

In SQL, AND and OR aren't functions; they're keywords that you use to combine conditions in the WHERE clause.

For example, we could filter the HumanResources.Department table for departments whose ID is less than or equal to 5 and the department group name is Research and Development with:

SELECT
    DepartmentID,
    Name,
    GroupName
FROM HumanResources.Department
WHERE DepartmentID <= 5 AND GroupName = 'Research and Development'
;
DepartmentID Name GroupName
1 Engineering Research and Development
2 Tool Design Research and Development

Tip

Combining conditions "sounds like" English, so the query above can be read as:

"Select the department ID, name, and group name from the HumanResources.Department table where the department ID is less than or equal to 5 and the group name is Research and Development".

Similarly, we could filter the HumanResources.Department table for departments whose group name is either Sales and Marketing or Research and Development using:

SELECT
    DepartmentID,
    Name,
    GroupName
FROM HumanResources.Department
WHERE GroupName = 'Sales and Marketing' OR GroupName = 'Research and Development'
;
DepartmentID Name GroupName
1 Engineering Research and Development
2 Tool Design Research and Development
3 Sales Sales and Marketing
4 Marketing Sales and Marketing
6 Research and Development Research and Development

It's common to use IN (and NOT IN) to streamline multiple OR conditions. For example, the previous query could be written as:

SELECT
    DepartmentID,
    Name,
    GroupName
FROM HumanResources.Department
WHERE GroupName IN ('Sales and Marketing', 'Research and Development')
;

Note that the IN keyword is followed by a list of values in brackets separated by commas.

You can combine AND and OR in the same WHERE clause, but it's a good idea to use brackets to make the order of operations clear. For example, the following query filters the HumanResources.Department table for departments whose ID is less than or equal to 5 and the department group name is Research and Development, or the department group name is ``:

SELECT
    DepartmentID,
    Name,
    GroupName
FROM HumanResources.Department
WHERE (DepartmentID <= 5 AND GroupName = 'Research and Development')
   OR DepartmentID = 10
;
DepartmentID Name GroupName
1 Engineering Research and Development
2 Tool Design Research and Development
10 Finance Executive General and Administration

Use IS (NOT) NULL to filter NULL values

Warning

We'll learn more about NULL values in the Data types section, but for now, we'll mention that NULL is a special value that you'll sometimes see in SQL which represents a missing value similar to how (blank) is used in Excel.

Instead of using = or != to filter NULL values, you need to use the special IS NULL or IS NOT NULL keywords.

For example, we could filter the HumanResources.EmployeeDepartmentHistory table for employees whose department end date is missing using:

SELECT
    BusinessEntityID,
    DepartmentID,
    StartDate,
    EndDate
FROM HumanResources.EmployeeDepartmentHistory
WHERE EndDate IS NULL
BusinessEntityID DepartmentID StartDate EndDate
1 16 2009-01-14 null
2 1 2008-01-31 null
3 1 2007-11-11 null
4 2 2010-05-31 null
5 1 2008-01-06 null

Further reading

Check out the official Microsoft documentation for more information on the WHERE clause at:

The video version of this content is also available at:

Additional comparison operators

There are additional comparison operators that you can use in the WHERE clause which are outside the scope of this course. These include but are not limited to the BETWEEN, LIKE, and EXISTS operators:

Error

This is a contrived example to show some of the additional comparison operators.

SELECT
    DepartmentID,
    Name,
    GroupName
FROM HumanResources.Department
WHERE DepartmentID BETWEEN 1 AND 5
   OR GroupName NOT LIKE '%and%'
;