Be able to use a single criterion, or multiple criteria to select subsets of data using a query

18 Databases – Selecting Data with Queries

What is a Query?

Think of a database as a huge digital library 📚. A query is like asking the librarian for a specific book or set of books. In SQL (the language most databases use), we write queries to select the data we need.

The SELECT Statement

The basic structure of a query is:

SELECT column1, column2, …
FROM table_name
WHERE condition;

SELECT tells the database which columns you want.

FROM tells it which table.

WHERE filters the rows based on a condition.

Single Criterion – One Condition

Suppose we have a table called Students:

To get all students who are in Grade A:

SELECT Name, Age
FROM Students
WHERE Grade = 'A';

This will return:

• Alice, 15
• Charlie, 15

Multiple Criteria – Combining Conditions

Sometimes you want to narrow down further. Use AND, OR, and NOT to combine conditions.

AND – All Conditions Must Be True

Students who are Grade A and Age 15:

SELECT Name
FROM Students
WHERE Grade = 'A' AND Age = 15;

OR – Any Condition Can Be True

Students who are Grade A or Age 16:

SELECT Name
FROM Students
WHERE Grade = 'A' OR Age = 16;

NOT – Exclude a Condition

All students who are NOT in Grade B:

SELECT Name
FROM Students
WHERE NOT Grade = 'B';

Comparison Operators

Use these to compare numeric values:

• = equals
• < less than
• > greater than
• <= less than or equal to
• >= greater than or equal to
• != not equal

Example: Students older than 15:

SELECT Name
FROM Students
WHERE Age > 15;

Special Operators – LIKE, IN, BETWEEN

LIKE is used for pattern matching (think of it as a wildcard search).

SELECT Name
FROM Students
WHERE Name LIKE 'A%';  

IN lets you specify a list of acceptable values.

SELECT Name
FROM Students
WHERE Grade IN ('A', 'B');

BETWEEN checks if a value lies within a range.

SELECT Name
FROM Students
WHERE Age BETWEEN 15 AND 16;

Putting It All Together – A Complete Example

Find the names of students who are either in Grade A or B, are older than 15, and whose names start with the letter C:

SELECT Name
FROM Students
WHERE (Grade = 'A' OR Grade = 'B')
  AND Age > 15
  AND Name LIKE 'C%';

Quick Practice

1. Write a query to list all students who are exactly 16 years old.
2. Modify the query to also show their grades.
3. Change the query to exclude students with Grade C.

💡 Tip: Think of each condition as a filter. The more filters you add, the smaller the group of results.

Key Takeaways

• Use SELECT to choose columns.
• Use WHERE to filter rows.
• Combine conditions with AND, OR, NOT.
• Use comparison operators for numeric data.
• Use LIKE, IN, and BETWEEN for more advanced filtering.

Happy querying! 🚀