SQL Basics
Querying Data
Filtering Data
Joining Tables
SQL Functions
Modifying Data
Defining Data
SQL WHERE
In this tutorial, you will learn how to filter rows based on specified conditions using the WHERE clause.
Once you know how to fetch your data using SELECT and FROM and the next step is to learn how to filter your data using WHERE clause.
In the previous tutorial, you learned how to retrieve all your rows from tables, but in real-world scenarios, we usually select only the rows which fulfill certain conditions like customers who come from a certain country.
Syntax
The basic syntax of the WHERE clause to filter the data returned by a query can be given with:
SELECT column_names
FROM table_name
WHERE condition
It’s real easy to read in a plain-English way: select some columns from a table, except that the results will only include rows where that fulfill my conditions.
- The clauses need to be in this order e SELECT FROM WHERE
- The WHERE clause appears immediately after the FROM clause
- In WHERE clause, you can specify one or more comparisons and logical operators.
Examples
To understand the WHERE statement in a better way, let’s look at the following customers table in our tutorial database:
| customer_id | first_name | last_name | country | score |
|---|---|---|---|---|
| 1 | Maria | Cramer | Germany | 350 |
| 2 | John | Steel | USA | 900 |
| 3 | Georg | Pipps | UK | 750 |
| 4 | Martin | Müller | Germany | 500 |
| 5 | Peter | Franken | USA | NULL |
Now, let’s check out some examples that demonstrate how it actually works.
We have the following task be to solve using SQL statements
The following SQL statement will return only german customers from the database
SELECT *
FROM customers
WHERE country = 'Germany'
After executing the above query, you’ll get the result set something like this:
| customer_id | first_name | last_name | country | score |
|---|---|---|---|---|
| 1 | Maria | Cramer | Germany | 350 |
| 4 | Martin | Müller | Germany | 500 |
As you can see the output contains only german customers.
The following SQL statement will return only customers with score greater than 500
SELECT *
FROM customers
WHERE score > 500
After executing the above query, you’ll get the result set something like this:
| customer_id | first_name | last_name | country | score |
|---|---|---|---|---|
| 2 | John | Steel | USA | 900 |
| 3 | Georg | Pipps | UK | 750 |
Operators in WHERE Clause
You can filter your results in a number of ways using comparison and logical operators, which you’ll learn about in the next tutorials. I summarized in the following table the most important ones.
Comparison Operators
| = | Equal |
| != or <> | Not equal |
| > | Greater than |
| < | Less than |
| >= | Greater than or equal to |
| <= | Less than or equal to |
Logical Operators
| AND | Return True if both conditions are True |
| OR | Return True if one of conditions is True |
| NOT | Reverse the result of any Boolean operator |
| IN | Return True if a value is in a set of values |
| BETWEEN | Return True if a value falls within a specific range |
| LIKE | Return True if a value matches a pattern |
