# 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.

```sql
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:

```sql
SELECT * FROM users;
```

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

```sql
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`:

```sql
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.

```sql
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.

```sql
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.

```sql
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`.

```sql
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.

```sql
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.

```sql
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.

```sql
SELECT * FROM users; -- Retrieve all user records
```

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

```sql
/*
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.