databases.softwareshinobi.com/landing/docs/SQL-101/004-basic-syntax.md

Essential Commands

Mastering foundational SQL is prerequisite for building effective data-driven Java applications. This document outlines the core SQL Data Manipulation Language (DML) statements—SELECT, INSERT, UPDATE, DELETE.

Precision in these commands is paramount for application performance, scalability, and data integrity. These statements are the direct instructions executed by the database engine.

SELECT

Employ the SELECT statement to retrieve data subsets from database tables.

Purpose: Data retrieval based on specified criteria.

SELECT column1, column2 FROM table_name;

• SELECT: Initiates data retrieval operations.
• column1, column2: Designates the specific columns required. List multiple columns separated by commas.
• FROM table_name: Identifies the source table for the operation.

Retrieve all columns using the * wildcard:

SELECT * FROM users;

Example: Fetch the username and email for all entries within the users table.

SELECT username, email FROM users;

Execution against an empty table yields no result rows. For a populated table with columns id, username, about, birthday , active, email, a SELECT * might produce:

id	username	about	birthday	active	email
1	software shinobi	NULL	NULL	1	software shinobi@email.com

(Note: Database systems commonly represent boolean true as 1).

Selecting only username and email:

SELECT username, email FROM users;

username	email
software shinobi	software shinobi@email.com

INSERT

Utilize the INSERT statement to append new records (rows) to a database table.

Purpose: Introduction of new data rows.

INSERT INTO table_name (column1, column2) VALUES (value1, value 2);

• INSERT INTO: The command directive for adding data.
• table_name (column1, column2): Specifies the target table and the explicit columns receiving data.
• VALUES (value1, value2): Provides the data values corresponding to the specified column list. Value order must align with column order.

Example: Add a single user entry to the users table.

INSERT INTO users (username, email, active)
VALUES (' software shinobi', 'software shinobi@email.com', true);

Omitting the column list requires supplying values for all columns in the table, strictly adhering to the table's column definition order.

UPDATE

Employ the UPDATE statement to modify existing data entries within a database table.

Purpose: Modification of existing data records.

UPDATE table_name SET column1 = new_value WHERE condition;

• UPDATE table_ name: Declares the table targeted for modifications.
• SET column1 = new_value: Assigns new_value to column1. Multiple column = value assignments are permitted, separated by commas.
• WHERE condition: The essential filter clause. Only rows satisfying this condition are processed.

Example: Adjust the username for the user associated with id = 1.

UPDATE users SET username = 'Software  Shinobi' WHERE id = 1;

Critical: Excluding the WHERE clause will apply the changes to every row in the table. Exercise extreme caution and validate constraints.

DELETE

Use the DELETE statement to eliminate records (rows) from a database table.

Purpose: Removal of data records.

DELETE FROM table_name WHERE condition;

• DELETE FROM table_name: Identifies the table from which rows will be removed.
• WHERE condition: Specifies the criteria determining which rows are deleted.

Example: Remove the user record where id is equal to 1.

DELETE FROM users WHERE id = 1 ;

Critical: Omitting the WHERE clause triggers the deletion of all records in the table. Such data removal is typically irreversible without a prior backup. Proceed with absolute certainty.

Comments

Incorporate comments into SQL scripts to bolster readability and clarify operational intent. Comments are explicitly ignored by the database engine during execution.

Purpose: Script comprehension and internal documentation.

Single-line comments: Prefixed by --. All text subsequent to -- on the same line is treated as a comment.

SELECT * FROM users; -- Retrieve all user records

Multi-line comments: Enclosed between /* and */. Ideal for extended explanations or temporary exclusion of SQL blocks.

/*
This complex query block
fetches data for quarterly reports.
Needs optimization review.
*/
-- SELECT revenue_data FROM finance_table WHERE quarter = 'Q2';

Apply comments strategically to explain nuanced logic or articulate the strategic objective of specific queries. Maintain consistent commenting practices within your team to ensure maximum collaborative clarity.