SQL Functions & Expressions Explained with Examples + Practice Questions

When to Use Functions & Expressions

• Use functions to transform or calculate data
• Use expressions to create new values from existing columns

These are commonly used in:

• Data analysis
• Reporting
• SQL interviews

Example Dataset

Table: employees

What are SQL Functions

Functions perform operations on data and return a result.

Common Types of Functions

1. String Functions

UPPER (convert to uppercase)

sql
SELECT UPPER(name) FROM employees;

LOWER (convert to lowercase)

sql
SELECT LOWER(name) FROM employees;

LENGTH (count characters)

sql
SELECT name, LENGTH(name) FROM employees;

1. Numeric Functions

Basic Arithmetic

sql
SELECT name, salary + 5000 AS new_salary
FROM employees;

1. Aggregate Functions (Preview)

sql
SELECT COUNT(*) FROM employees;

Counts total rows.

What are Expressions

Expressions combine:

• columns
• values
• operators
• functions

to produce new results.

Examples:

Increase Salary

sql
SELECT name, salary * 1.10 AS increased_salary
FROM employees;

Concatenate Values

sql
SELECT name || ' works in ' || department AS info
FROM employees;

Conditional Expression (CASE)

sql
SELECT name,
CASE
    WHEN salary > 60000 THEN 'High'
    WHEN salary >= 50000 THEN 'Medium'
    ELSE 'Low'
END AS salary_level
FROM employees;

SQL LIKE vs Expression Example

sql
SELECT name, salary * 2 AS double_salary
FROM employees
WHERE name LIKE 'A%';

Combines:

• filtering
• expressions

Key Concepts

• Functions modify or calculate values
• Expressions create new computed columns
• Use AS to rename output columns
• Expressions are heavily used in real-world queries

Common Questions (FAQ)

What are SQL functions

SQL functions are built-in operations that perform calculations or transformations on data.

What is an expression in SQL

An expression is a combination of columns, values, and operators used to compute a result.

What is CASE in SQL

CASE is used for conditional logic, similar to if-else statements.

Can we use functions in WHERE

Yes, but it may impact performance in large datasets.

Internal Learning Links

• Learn SELECT basics
• Learn WHERE filtering
• Learn Aggregations

Practice CTA

Practice SQL function-based questions on Schoolabe to strengthen your query-building skills.