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SQL

SQL Queries

SQL Queries

SQL (Structured Query Language) is a powerful tool for managing and manipulating databases. It allows users to perform various operations such as retrieving, updating, deleting, and inserting data in a structured format.

1. SELECT Statement

The SELECT command retrieves specific records from a database table based on given conditions.

Syntax:

SELECT column1, column2, ...  
FROM table_name  
WHERE condition;
;

Retrieve all employees whose salary exceeds 50,000.

Example:

SELECT name, salary  
FROM employees  
WHERE salary > 50000;

2. INSERT INTO Statement

This command adds new data into a table.

Syntax:

INSERT INTO table_name (column1, column2, ...)  
VALUES (value1, value2, ...);

Insert a new student into the students table.

Example:

INSERT INTO students (id, name, age, course)  
VALUES (101, 'John Doe', 22, 'Computer Science');

3. UPDATE Statement

Used to modify existing records in a table.

Syntax:

UPDATE table_name  
SET column1 = value1, column2 = value2, ...  
WHERE condition;

Update the salary of an employee named "Alice".

Example:

UPDATE employees  
SET salary = 75000  
WHERE name = 'Alice';

4. DELETE Statement

Removes records from a table based on specified conditions.

Syntax:

DELETE FROM table_name  
WHERE condition;

Delete records of students who failed the exam.

Example:

DELETE FROM students  
WHERE marks < 40;

5. CREATE TABLE Statement

Defines a new table structure.

Syntax:

CREATE TABLE table_name (  
    column1 datatype constraints,  
    column2 datatype constraints,  
    ...  
);

Create a customers table.

Example:

CREATE TABLE customers (  
    id INT PRIMARY KEY,  
    name VARCHAR(100),  
    email VARCHAR(255) UNIQUE,  
    age INT  
);

6. ALTER TABLE Statement

Modifies an existing table’s structure.

Syntax (Adding a Column):

ALTER TABLE table_name  
ADD column_name datatype;

Add a phone_number column to the customers table.

Example:

ALTER TABLE customers  
ADD phone_number VARCHAR(15);

7. DROP TABLE Statement

Deletes an entire table along with its data.

Syntax:

DROP TABLE table_name;

Remove the backup_data table permanently.

Example:

  DROP TABLE backup_data;

8. TRUNCATE TABLE Statement

Removes all records from a table but keeps its structure.

Syntax:

TRUNCATE TABLE table_name;

Delete all rows from the orders table but retain its schema.

Example:

TRUNCATE TABLE orders;

9. JOIN Clause

Used to combine rows from multiple tables based on a related column.

Syntax (INNER JOIN):

SELECT table1.column1, table2.column2  
FROM table1  
INNER JOIN table2 ON table1.common_column = table2.common_column;

Retrieve employee names along with their department names.

Example:

SELECT employees.name, departments.department_name  
FROM employees  
INNER JOIN departments ON employees.department_id = departments.id;

10. GROUP BY Clause

Groups rows that share a common value and applies aggregate functions.

Syntax:

SELECT column_name, aggregate_function(column_name)  
FROM table_name  
GROUP BY column_name;

Find the total salary paid to each department.

Example:

SELECT department_id, SUM(salary)  
FROM employees  
GROUP BY department_id;

11. HAVING Clause

Filters grouped records based on aggregate function conditions.

Syntax:

SELECT column_name, aggregate_function(column_name)  
FROM table_name  
GROUP BY column_name  
HAVING condition;

Find departments where total salary exceeds 100,000.

Example:

SELECT department_id, SUM(salary)  
FROM employees  
GROUP BY department_id  
HAVING SUM(salary) > 100000;

12. ORDER BY Clause

Sorts the result set in ascending (ASC) or descending (DESC) order.

Syntax:

SELECT column_name  
FROM table_name  
ORDER BY column_name ASC|DESC;

List students in descending order of marks.

Example:

SELECT name, marks  
FROM students  
ORDER BY marks DESC;

13. DISTINCT Keyword

Eliminates duplicate values from a query result.

Syntax:

SELECT DISTINCT column_name  
FROM table_name;

Find unique job titles in a company.

Example:

SELECT DISTINCT job_title  
FROM employees;

14. LIMIT Clause

Restricts the number of records returned by a query.

Syntax:

SELECT column_name  
FROM table_name  
LIMIT number;

Retrieve the top 5 highest-paid employees.

Example:

SELECT name, salary  
FROM employees  
ORDER BY salary DESC  
LIMIT 5;

15. CASE Statement

Implements conditional logic in queries.

Syntax:

SELECT column_name,  
    CASE  
        WHEN condition1 THEN result1  
        WHEN condition2 THEN result2  
        ELSE default_result  
    END  
FROM table_name;

Categorize employees based on salary.

Example:

 SELECT name, salary,  
  CASE  
      WHEN salary > 80000 THEN 'High'  
      WHEN salary BETWEEN 50000 AND 80000 THEN 'Medium'  
      ELSE 'Low'  
  END AS salary_category  
FROM employees;
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