Software Shinobi
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5.1 KiB
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141 lines
5.1 KiB
Markdown
# Essential Commands
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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`.
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Precision in these commands is paramount for application performance, scalability, and data integrity. These statements are the direct instructions executed by the database engine.
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## SELECT
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Employ the `SELECT` statement to retrieve data subsets from database tables.
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**Purpose:** Data retrieval based on specified criteria.
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```sql
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SELECT column1, column2 FROM table_name;
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```
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* `SELECT`: Initiates data retrieval operations.
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* `column1, column2`: Designates the specific columns required. List multiple columns separated by commas.
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* `FROM table_name`: Identifies the source table for the operation.
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Retrieve all columns using the `*` wildcard:
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```sql
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SELECT * FROM users;
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```
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**Example:** Fetch the `username` and `email` for all entries within the `users` table.
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```sql
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SELECT username, email FROM users;
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```
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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:
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| id | username | about | birthday | active | email |
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|----|----------------|-------|----------|--------|----------------------|
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| 1 | software shinobi | NULL | NULL | 1 | software shinobi@email.com |
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*(Note: Database systems commonly represent boolean `true` as `1`)*.
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Selecting only `username` and `email`:
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```sql
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SELECT username, email FROM users;
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```
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| username | email |
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|----------------|----------------------|
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| software shinobi | software shinobi@email.com |
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## INSERT
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Utilize the `INSERT` statement to append new records (rows) to a database table.
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**Purpose:** Introduction of new data rows.
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```sql
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INSERT INTO table_name (column1, column2) VALUES (value1, value 2);
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```
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* `INSERT INTO`: The command directive for adding data.
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* `table_name (column1, column2)`: Specifies the target table and the explicit columns receiving data.
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* `VALUES (value1, value2)`: Provides the data values corresponding to the specified column list. Value order must align with column order.
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**Example:** Add a single user entry to the `users` table.
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```sql
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INSERT INTO users (username, email, active)
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VALUES (' software shinobi', 'software shinobi@email.com', true);
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```
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Omitting the column list requires supplying values for *all* columns in the table, strictly adhering to the table's column definition order.
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## UPDATE
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Employ the `UPDATE` statement to modify existing data entries within a database table.
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**Purpose:** Modification of existing data records.
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```sql
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UPDATE table_name SET column1 = new_value WHERE condition;
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```
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* `UPDATE table_ name`: Declares the table targeted for modifications.
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* `SET column1 = new_value`: Assigns `new_value` to `column1`. Multiple `column = value` assignments are permitted, separated by commas.
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* `WHERE condition`: The essential filter clause. Only rows satisfying this condition are processed.
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**Example:** Adjust the username for the user associated with `id = 1`.
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```sql
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UPDATE users SET username = 'Software Shinobi' WHERE id = 1;
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```
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**Critical:** Excluding the `WHERE` clause will apply the changes to *every* row in the table. Exercise extreme caution and validate constraints.
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## DELETE
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Use the `DELETE` statement to eliminate records (rows) from a database table.
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**Purpose:** Removal of data records.
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```sql
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DELETE FROM table_name WHERE condition;
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```
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* `DELETE FROM table_name`: Identifies the table from which rows will be removed.
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* `WHERE condition`: Specifies the criteria determining which rows are deleted.
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**Example:** Remove the user record where `id` is equal to 1.
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```sql
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DELETE FROM users WHERE id = 1 ;
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```
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**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.
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## Comments
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Incorporate comments into SQL scripts to bolster readability and clarify operational intent. Comments are explicitly ignored by the database engine during execution.
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**Purpose:** Script comprehension and internal documentation.
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**Single-line comments:** Prefixed by `--`. All text subsequent to `--` on the same line is treated as a comment.
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```sql
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SELECT * FROM users; -- Retrieve all user records
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```
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**Multi-line comments:** Enclosed between `/*` and `*/`. Ideal for extended explanations or temporary exclusion of SQL blocks.
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```sql
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/*
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This complex query block
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fetches data for quarterly reports.
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Needs optimization review.
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*/
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-- SELECT revenue_data FROM finance_table WHERE quarter = 'Q2';
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```
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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.
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