SQL

SQL

Standard language for storing, manipulating and retrieving data in databases.

When to Use

• Relational data modeling
• Complex queries and aggregations
• Data integrity enforcement (ACID)
• Standardized data access

Quick Start

-- Create Table
CREATE TABLE users (
    id SERIAL PRIMARY KEY,
    name VARCHAR(100),
    email VARCHAR(100) UNIQUE
);

-- Insert Data
INSERT INTO users (name, email) VALUES ('Alice', 'alice@example.com');

-- Query Data
SELECT * FROM users WHERE name = 'Alice';

Core Concepts

DDL (Data Definition Language)

Commands to define database schemas (CREATE, ALTER, DROP).

DML (Data Manipulation Language)

Commands to manipulate data (SELECT, INSERT, UPDATE, DELETE).

Joins

Combining rows from two or more tables based on a related column.

• INNER JOIN: Matches in both tables.
• LEFT JOIN: All from left, matches from right.

Best Practices

Do:

• Use parameterized queries (prevent SQL Injection)
• Index columns used in WHERE and JOIN clauses
• Use transactions for atomic operations

Don't:

• Use SELECT * in production (fetch only what you need)
• Store logic in triggers if possible (hard to debug)
• Ignore execution plans for slow queries

References

• PostgreSQL Documentation

Source: G1Joshi/Agent-Skills — distributed by TomeVault.