WHERE
The WHERE clause allows you to specify different conditions so that you could filter out the data and get a specific result set.
You would add the WHERE clause after the FROM clause.
The syntax would look like this:
SELECT column_name FROM table_name WHERE column=some_value;
WHERE Clause example
If we take the example users table from the last chapter, let's say that we wanted to get only the active users. The SQL statement would look like this:
SELECT DISTINCT username,email,active FROM users WHERE active=true;
Output:
+----------+---------------+--------+
| username | email | active |
+----------+---------------+--------+
| bobby | [email protected] | 1 |
| tony | [email protected] | 1 |
+----------+---------------+--------+
As you can see, we are only getting tony and bobby back as their active column is true or 1. If we wanted to get the inactive users, we would have to change the WHERE clause and set the active to false:
+----------+---------------+--------+
| username | email | active |
+----------+---------------+--------+
| devdojo | [email protected] | 0 |
+----------+---------------+--------+
As another example, let's say that we wanted to select all users with the username bobby. The query, in this case, would be:
SELECT username,email,active FROM users WHERE username='bobby';
The output would look like this:
+----------+---------------+--------+
| username | email | active |
+----------+---------------+--------+
| bobby | [email protected] | 1 |
| bobby | [email protected] | 1 |
+----------+---------------+--------+
We are getting 2 entries back as we have 2 users in our database with the username bobby.
Operators
In the example, we used the = operator, which checks if the result set matches the value that we are looking for.
A list of popular operators are:
-
!=: Not equal operator -
>: Greater than -
>=: Greater than or equal operator -
<: Less than operator -
<=: Less than or equal operator
For more information about other available operators, make sure to check the official documentation here.
AND keyword
In some cases, you might want to specify multiple criteria. For example, you might want to get all users that are active, and the username matches a specific value. This could be achieved with the AND keyword.
Syntax:
SELECT * FROM users WHERE username='bobby' AND active=true;
The result set would contain the data that matches both conditions. In our case, the output would be:
+----+----------+-------+----------+--------+---------------+
| id | username | about | birthday | active | email |
+----+----------+-------+----------+--------+---------------+
| 2 | bobby | NULL | NULL | 1 | [email protected] |
| 5 | bobby | NULL | NULL | 1 | [email protected] |
+----+----------+-------+----------+--------+---------------+
If we were to change the AND statement to active=false, we would not get any results back as none of the entries in our database match that condition:
SELECT * FROM users WHERE username='bobby' AND active=false;
-- Output:
Empty set (0.01 sec)
OR keyword
In some cases, you might want to specify multiple criteria. For example, you might want to get all users that are active, or their username matches a specific value. This could be achieved with the OR keyword.
As with any other programming language, the main difference between AND and OR is that with AND, the result would only return the values that match the two conditions, and with OR, you would get a result that matches either of the conditions.
For example, if we were to run the same query as above but change the AND to OR, we would get all users that have the username bobby and also all users that are not active:
SELECT * FROM users WHERE username='bobby' OR active=false;
Output:
+----+----------+-------+----------+--------+---------------+
| id | username | about | birthday | active | email |
+----+----------+-------+----------+--------+---------------+
| 2 | bobby | NULL | NULL | 1 | [email protected] |
| 3 | devdojo | NULL | NULL | 0 | [email protected] |
| 5 | bobby | NULL | NULL | 1 | [email protected] |
| 6 | devdojo | NULL | NULL | 0 | [email protected] |
+----+----------+-------+----------+--------+---------------+
LIKE operator
Unlike the = operator, the LIKE operator allows you to do wildcard matching similar to the * symbol in Linux.
For example, if you wanted to get all users that have the y letter in them, you would run the following:
SELECT * FROM users WHERE username LIKE '%y%';
Output
+----+----------+-------+----------+--------+---------------+
| id | username | about | birthday | active | email |
+----+----------+-------+----------+--------+---------------+
| 2 | bobby | NULL | NULL | 1 | [email protected] |
| 4 | tony | NULL | NULL | 1 | [email protected] |
+----+----------+-------+----------+--------+---------------+
As you can see, we are getting only tony and bobby but not devdojo as there is no y in devdojo.
This is quite handy when you are building some search functionality for your application.
IN operator
The IN operator allows you to provide a list expression and would return the results that match that list of values.
For example, if you wanted to get all users that have the username bobby and devdojo, you could use the following:
SELECT * FROM users WHERE username IN('bobby', 'devdojo');
Output:
+----+----------+-------+----------+--------+---------------+
| id | username | about | birthday | active | email |
+----+----------+-------+----------+--------+---------------+
| 2 | bobby | NULL | NULL | 1 | [email protected] |
| 3 | devdojo | NULL | NULL | 0 | [email protected] |
| 5 | bobby | NULL | NULL | 1 | [email protected] |
| 6 | devdojo | NULL | NULL | 0 | [email protected] |
+----+----------+-------+----------+--------+---------------+
This allows you to simplify your WHERE expression so that you don't have to add numerous OR statements.
IS operator
If you were to run SELECT * FROM users WHERE about=NULL; you would get an empty result set as the = operator can't be used to check for NULL values. Instead, you would need to use the IS operator instead.
The IS operator is only used to check NULL values, and the syntax is the following:
SELECT * FROM users WHERE about IS NULL;
If you wanted to get the results where the value is not NULL, you just need to change IS to IS NOT:
SELECT * FROM users WHERE about IS NOT NULL;
Conclusion
In this chapter, you've learned how to use the WHERE clause with different operators to get different type of results based on the parameters that you provide.
In the next chapter, we will learn how to order the result set.