Retrieving and Filtering Data in SQL

 

Retrieving and Filtering Data in SQL

We introduced SQL and why it’s essential. Now it’s time to get hands-on! In this blog, we’ll explore how to retrieve specific data from a database and filter it based on conditions. By the end, you’ll be writing your own SQL queries like a pro.

1. Retrieving Data with SELECT

The SELECT statement is the foundation of SQL queries. It tells the database which columns you want to see.

SELECT first_name, last_name, department
FROM employees;

  • Retrieves the first_name, last_name, and department columns from the employees table.

  • If you want all columns, use *:

SELECT *
FROM employees;

2. Filtering Data with WHERE
Often, you don’t want all the rows. The WHERE clause filters data based on conditions.
SELECT first_name, last_name, department
FROM employees
WHERE department = 'Sales';
  • Only shows employees in the Sales department.
  • Conditions can use operators like =, >, <, >=, <=, !=.
Example: Filtering by Number
SELECT first_name, salary
FROM employees
WHERE salary > 50000;
  • Returns employees earning more than 50,000.
3. Combining Conditions
Use AND / OR to combine multiple conditions:
SELECT first_name, last_name, department, salary
FROM employees
WHERE department = 'Sales' AND salary > 50000;
  • Returns employees in Sales with salary over 50,000.
SELECT first_name, last_name, department
FROM employees
WHERE department = 'Sales' OR department = 'Marketing';
  • Returns employees in Sales or Marketing.
4. Sorting Data with ORDER BY
The ORDER BY clause lets you sort results:
SELECT first_name, last_name, salary
FROM employees
ORDER BY salary DESC;
  • Sorts employees by salary in descending order.
  • Use ASC for ascending (default).
5. Limiting Results
Sometimes, you only want the top few results:
SELECT first_name, last_name, salary
FROM employees
ORDER BY salary DESC
LIMIT 5;
  • Shows the top 5 highest-paid employees.
6. Using LIKE for Pattern Matching
LIKE allows you to search for patterns:
SELECT first_name, last_name
FROM employees
WHERE first_name LIKE 'J%';
  • Returns names starting with J (John, Jane…).
  • % – any sequence of characters.
  • _ – single character wildcard.
By now, you can:
  • Select specific columns.
  • Filter rows using WHERE.
  • Combine conditions with AND / OR.
  • Sort results using ORDER BY.
  • Search with patterns using LIKE.
These are the building blocks of all SQL queries, and mastering them will make analyzing data much easier.

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