To update schemas and data in a Relational Database (RAG), you’ll need to understand the specific SQL commands and procedures necessary for altering schemas and updating the data within the database. Below is an informative guide, including examples and sources, on how to perform these tasks.
Updating the schema of a relational database involves modifying the structure of database objects such as tables, indexes, and columns. Here are some common SQL commands used to update schemas.
1. ALTER TABLE: This command allows you to add, delete, or modify columns in an existing table.
- Adding a Column:
\`\`\`sql
ALTER TABLE employees ADD COLUMN department VARCHAR;
\`\`\`
This command adds a new column named `department` to the `employees` table.
- Deleting a Column: \`\`\`sql ALTER TABLE employees DROP COLUMN department; \`\`\` This command removes the `department` column from the `employees` table.
- Modifying a Column: \`\`\`sql ALTER TABLE employees MODIFY COLUMN department VARCHAR; \`\`\` This command changes the data type of the `department` column to `VARCHAR`.
1. Creating Indexes: \`\`\`sql CREATE INDEX idx_employee_name ON employees (name); \`\`\` This command creates an index on the `name` column of the `employees` table, which can improve query performance.
1. Dropping Indexes: \`\`\`sql DROP INDEX idx_employee_name ON employees; \`\`\` This command removes the index named `idx_employee_name` from the `employees` table.
To update the actual data within a relational database, you use Data Manipulation Language (DML) commands such as `UPDATE`, `INSERT`, and `DELETE`.
1. Updating Data: \`\`\`sql UPDATE employees SET salary = salary \* 1.10 WHERE department = ‘Sales’; \`\`\` This command increases the salary of all employees in the Sales department by 10%.
1. Inserting Data: \`\`\`sql INSERT INTO employees (name, department, salary) VALUES (‘John Doe’, ‘Marketing’, 75000); \`\`\` This command inserts a new record into the `employees` table.
1. Deleting Data: \`\`\`sql DELETE FROM employees WHERE name = ‘John Doe’; \`\`\` This command removes the record of the employee named `John Doe` from the `employees` table.
Imagine an application for a company’s HR department that tracks employees. Initially, the `employees` table didn’t have a `start_date` column. To add this new column, you would run:
```
ALTER TABLE employees ADD COLUMN start_date DATE;
```
Once the schema is updated, you might want to add start dates to existing records:
```
UPDATE employees SET start_date=‘2020-01-15’ WHERE name=‘Alice Smith’;
UPDATE employees SET start_date=‘2019-03-22’ WHERE name=‘Bob Johnson’;
```
1. Books:
- SQL in 10 Minutes, Sams Teach Yourself by Ben Forta
- Learning SQL by Alan Beaulieu
1. Online Courses:
- Coursera: SQL for Data Science by University of California, Davis
- Udemy: The Complete SQL Bootcamp by Jose Portilla
1. Official Documentation:
- [MySQL Documentation](https://dev.mysql.com/doc/)
- [PostgreSQL Documentation](https://www.postgresql.org/docs/)
By using these commands and resources, you can efficiently update schemas and data in a relational database, ensuring that your database structure and content remain relevant and optimized for your specific application needs.
