SQL: Expressions

In SQL, expressions are combinations of values, operators, and functions that produce a single value.
They are used in queries to compute, filter, transform, or return results from database data.

1. Types of SQL Expressions

a) Constant Expressions

Return a fixed value; do not depend on table data.

SELECT 100 AS FixedValue;
SELECT 'Hello SQL' AS Message;

b) Column Expressions

Use column names directly.

SELECT first_name, last_name FROM employees;

c) Arithmetic Expressions

Perform mathematical calculations.
Operators: +, -, *, /, %

SELECT salary, salary * 0.1 AS Bonus
FROM employees;

d) String Expressions

Manipulate or combine text.

SELECT CONCAT(first_name, ' ', last_name) AS FullName
FROM employees;

e) Comparison Expressions

Used in conditions to compare values.
Operators: =, !=, <>, <, >, <=, >=

SELECT * FROM employees
WHERE salary > 50000;

f) Logical Expressions

Combine conditions to return TRUE/FALSE.
Operators: AND, OR, NOT

SELECT * FROM employees
WHERE department = 'Sales' AND salary > 40000;

g) Case Expressions (Conditional)

Return values based on conditions.

SELECT name,
       CASE 
           WHEN marks >= 90 THEN 'A'
           WHEN marks >= 75 THEN 'B'
           ELSE 'C'
       END AS Grade
FROM students;

h) Aggregate Expressions

Use aggregate functions to return a single computed value.

SELECT AVG(salary) AS AvgSalary
FROM employees;

i) Date and Time Expressions

Work with date/time values.

SELECT NOW() AS CurrentTime;
SELECT DATEDIFF('2025-08-08', '2025-01-01') AS DaysDifference;

2. Where Expressions Are Used in SQL

SELECT clause → to calculate or display computed values.
WHERE clause → to filter rows.
ORDER BY clause → to sort results.
GROUP BY and HAVING clauses → for aggregated results.
UPDATE statements → to modify column values.

Example:

UPDATE employees
SET salary = salary * 1.05
WHERE department = 'IT';

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