Using the WHERE Clause

In databases, tables typically hold substantial volumes of data. However, frequently we're interested in retrieving specific portions of the data rather than the entirety. To accomplish this, we need to define the conditions for data retrieval, which are referred to as filtering criteria.

Data is filtered using a WHERE clause specifying the search criteria in a SELECT statement. The WHERE clause appears immediately after the table name.

Let's see an example:

Explanation: The SELECT statement gets 2 columns from the country table and returns only rows with the continent value 'Europe'.

Note
Note that we need to enclose the value in single quotation marks('').

Clause Position

When we use the ORDER BY and WHERE clauses, we ensure the ORDER BY comes after the WHERE clause. Let's see the following example:

Task

Write an SQL query to retrieve the id, name, and region columns from the country table (please retrieve these columns in this order), returning only rows with the 'North America' continent.

Please note that North America should be correctly capitalized, and north america is not the same. So, be careful and write it as North America.

Here's a short example of the country table:

id	name	continent	region	surfacearea	capital	population
1	Japan	Asia	Eastern Asia	377829	Tokyo	126714000
2	Latvia	Europe	NULL	64589	Riga	2424200
3	Mexico	North America	Central America	1958201	Mexico City	98881000
...	...	...	...	...	...	...
15	Malta	Europe	Southern Europe	316	Valletta	380200

Everything was clear?

Section 3. Chapter 1

Course Content

Introduction to SQL

1. Retrieving Data

Retrieving Individual Columns Retrieving Multiple Columns Retrieving All Columns Retrieving Distinct Rows Limiting Results Challenge: Find the Population of the Countries Challenge: Find All Countries Challenge: Find All Countries With Their IDs Challenge: Find Country Capitals Challenge: Find the Regions in Which All Countries Are Located

2. Sorting Retrieved Data

Sorting Data Challenge: Population Sort Sorting by Multiple Columns Challenge: Dual-Sort Query Specifying Sort Direction Challenge: Sort the Countries by Region and Capital Challenge: Sort Capitals in Descending Order Challenge: Sort Countries in Ascending Order Challenge: Find Countries, Their IDs, and Their Populations Challenge: Find Countries, IDs, Populations, Regions, and Sort Them Challenge: Find All Continents and Sort Them in Ascending Order

3. Filtering Data

Using the WHERE Clause Challenge: North America Population Sort The WHERE Clause Operators Challenge: Population Threshold Query Checking for a Range of Values Checking for NO Value

4. Advanced Data Filtering

Using the AND Operator Challenge: Europe Region Sort Using the OR operator Challenge: America Regions Filter Using the NOT Operator Challenge: Non-Europe Capitals Sort

5. Aggregate Functions

The AVG() Function Challenge: Large Surface Area Average The COUNT() Function Challenge: Continental Count The MAX() Function The MIN() Function Challenge: Surface Area Selection The SUM() Function