Oracle SQL Cheatsheet

Quick reference guide for Oracle database operations

Introduction to SQL

SQL (Structured Query Language) is the standard language for managing relational databases.

SQL Command Types

DDL: Data Definition Language (CREATE, ALTER, DROP)
DML: Data Manipulation Language (INSERT, UPDATE, DELETE)
DQL: Data Query Language (SELECT)
DCL: Data Control Language (GRANT, REVOKE)
TCL: Transaction Control Language (COMMIT, ROLLBACK)

Basic Example

SELECT * FROM customers 
WHERE country = 'USA' 
ORDER BY last_name 
FETCH FIRST 10 ROWS ONLY;

Setting Up Oracle Environment

Installation

Oracle Database Express Edition (XE)
SQL Developer (GUI)
Oracle Instant Client

Online Tools

Oracle Live SQL
DB Fiddle (Oracle mode)
SQL Fiddle (Oracle mode)

IDEs

Oracle SQL Developer
TOAD for Oracle
PL/SQL Developer
DBeaver

Schema and Table Operations

User/Schema Creation

CREATE USER my_user IDENTIFIED BY password;
GRANT CONNECT, RESOURCE TO my_user;
ALTER SESSION SET CURRENT_SCHEMA = my_user;

Table Creation

CREATE TABLE employees (
  id NUMBER PRIMARY KEY,
  name VARCHAR2(100) NOT NULL,
  email VARCHAR2(100) UNIQUE,
  hire_date DATE DEFAULT SYSDATE,
  salary NUMBER(10,2) CHECK (salary > 0)
);

CREATE SEQUENCE emp_seq START WITH 1;

Common Data Types

NUMBER: Numeric values
VARCHAR2(n): Variable string

DATE: Date and time
TIMESTAMP: Precise date/time

CHAR(n): Fixed-length string
CLOB: Large text data

Basic SQL Queries

Filtering Data

SELECT product_name, price
FROM products
WHERE price > 100 
  AND category = 'Electronics'
ORDER BY price DESC
FETCH FIRST 5 ROWS ONLY;

Sorting & Limiting

SELECT * FROM customers
ORDER BY last_name ASC, 
         first_name ASC
OFFSET 20 ROWS FETCH NEXT 10 ROWS ONLY;

Modifying Data (DML)

INSERT

INSERT INTO employees 
(id, name, email, salary)
VALUES 
(emp_seq.NEXTVAL, 'John Doe', 'john@example.com', 50000);

UPDATE

UPDATE employees
SET salary = salary * 1.1
WHERE department = 'Sales';

DELETE

DELETE FROM products
WHERE discontinued = 1;

Advanced Filtering

IN, BETWEEN, LIKE

SELECT * FROM products
WHERE category IN ('Electronics', 'Books')
  AND price BETWEEN 50 AND 200
  AND product_name LIKE '%Phone%';

NULL Handling

SELECT * FROM customers
WHERE phone IS NOT NULL;

UPDATE employees
SET manager_id = NULL
WHERE manager_id = 10;

Working with Multiple Tables

INNER JOIN

SELECT o.order_id, c.customer_name
FROM orders o
INNER JOIN customers c 
  ON o.customer_id = c.customer_id;

LEFT JOIN

SELECT e.name, d.department_name
FROM employees e
LEFT JOIN departments d
  ON e.dept_id = d.dept_id;

Self Join Example

SELECT e1.name AS employee, 
       e2.name AS manager
FROM employees e1
LEFT JOIN employees e2
  ON e1.manager_id = e2.employee_id;

Aggregation and Grouping

Aggregate Functions

SELECT 
  COUNT(*) AS total_products,
  AVG(price) AS avg_price,
  MIN(price) AS min_price,
  MAX(price) AS max_price,
  SUM(stock) AS total_stock
FROM products;

GROUP BY & HAVING

SELECT department_id, 
  COUNT(*) AS emp_count,
  AVG(salary) AS avg_salary
FROM employees
GROUP BY department_id
HAVING COUNT(*) > 5
ORDER BY avg_salary DESC;

Key Differences

WHERE filters before grouping
HAVING filters after grouping
Non-aggregated columns in SELECT must be in GROUP BY
Aggregate functions can't be used in WHERE

Subqueries

Single-Row Subquery

SELECT name, salary
FROM employees
WHERE salary > (
  SELECT AVG(salary)
  FROM employees
);

Multi-Row Subquery

SELECT product_name, price
FROM products
WHERE category_id IN (
  SELECT category_id
  FROM categories
  WHERE department = 'Electronics'
);

Correlated Subquery

SELECT e.name, e.salary, e.department_id
FROM employees e
WHERE salary > (
  SELECT AVG(salary)
  FROM employees
  WHERE department_id = e.department_id
);

Views and Indexes

Creating Views

CREATE OR REPLACE VIEW high_paid_employees AS
SELECT id, name, salary, department
FROM employees
WHERE salary > 100000
WITH CHECK OPTION;

SELECT * FROM high_paid_employees;

Creating Indexes

CREATE INDEX idx_employee_name 
ON employees(name);

CREATE UNIQUE INDEX idx_customer_email
ON customers(email);

CREATE INDEX idx_order_dates
ON orders(order_date, shipped_date);

Index Best Practices

Index columns frequently used in WHERE, JOIN, ORDER BY
Avoid over-indexing (slows down INSERT/UPDATE)
Consider composite indexes for multi-column queries
Monitor index usage and remove unused indexes

Basic SQL Functions

String Functions

SELECT 
  first_name || ' ' || last_name AS full_name,
  UPPER(email) AS email_upper,
  SUBSTR(phone, 1, 3) AS area_code,
  LENGTH(address) AS addr_length
FROM customers;

Date Functions

SELECT 
  SYSDATE AS current_datetime,
  TRUNC(order_date) AS order_date_only,
  due_date - SYSDATE AS days_remaining,
  ADD_MONTHS(hire_date, 12) AS anniversary
FROM orders;

Math Functions

SELECT 
  ROUND(price * 1.1, 2) AS price_with_tax,
  ABS(discount) AS absolute_discount,
  MOD(quantity, 10) AS remainder,
  CEIL(weight) AS rounded_weight
FROM products;

Transactions and ACID

Transaction Control

SET TRANSACTION NAME 'FundsTransfer';

UPDATE accounts 
SET balance = balance - 100
WHERE id = 123;

UPDATE accounts 
SET balance = balance + 100
WHERE id = 456;

COMMIT;
-- or ROLLBACK if error occurs

ACID Properties

Atomicity: All or nothing execution
Consistency: Valid state transitions
Isolation: Concurrent transactions don't interfere
Durability: Committed changes persist

PL/SQL Procedures & Functions

Stored Procedure

CREATE OR REPLACE PROCEDURE update_salary(
  emp_id IN NUMBER,
  increase_percent IN NUMBER
)
AS
BEGIN
  UPDATE employees
  SET salary = salary * (1 + increase_percent/100)
  WHERE id = emp_id;
  
  COMMIT;
END;
/

EXEC update_salary(101, 10);

PL/SQL Function

CREATE OR REPLACE FUNCTION get_employee_age(
  emp_id IN NUMBER
) RETURN NUMBER
AS
  age NUMBER;
BEGIN
  SELECT FLOOR(MONTHS_BETWEEN(SYSDATE, birth_date)/12)
  INTO age
  FROM employees
  WHERE id = emp_id;
  
  RETURN age;
END;
/

SELECT name, get_employee_age(id) AS age
FROM employees;

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