To interact with a Relational Algebra Query (RAG) database, the primary query language used is Structured Query Language, commonly known as SQL. SQL is a standardized language specifically designed for managing and manipulating relational databases. Developed in the 1970s, SQL has since become the dominant language for relational database management systems (RDBMS).
SQL provides a comprehensive set of commands that allow users to perform various tasks such as querying data, updating records, managing schema, and controlling access to the data. Some key components and commands in SQL include:
1. Data Query Language (DQL) – For querying data, the primary command used is `SELECT`. This command allows users to retrieve data from one or more tables and apply various filters and sorting options. For example: \`\`\`sql SELECT name, age FROM students WHERE grade = ‘A’; \`\`\` This query selects the `name` and `age` of students whose grade is ‘A’.
1. Data Manipulation Language (DML) – This includes commands to insert, update, and delete data. Examples include:
- `INSERT`: Adds new records to a table.
\`\`\`sql
INSERT INTO students (name, age, grade) VALUES (‘John Doe’, 18, ‘A’);
\`\`\`
- `UPDATE`: Modifies existing records.
\`\`\`sql
UPDATE students SET age = 19 WHERE name = ‘John Doe’;
\`\`\`
- `DELETE`: Removes records from a table.
\`\`\`sql
DELETE FROM students WHERE name = ‘John Doe’;
\`\`\`
1. Data Definition Language (DDL) – Used to define, alter, and manage schemas. Commands include:
- `CREATE`: Creates new tables, views, or other database objects.
\`\`\`sql
CREATE TABLE students (
id INT PRIMARY KEY,
name VARCHAR,
age INT,
grade CHAR
);
\`\`\`
- `ALTER`: Modifies an existing database object.
\`\`\`sql
ALTER TABLE students ADD COLUMN email VARCHAR;
\`\`\`
- `DROP`: Deletes existing database objects.
\`\`\`sql
DROP TABLE students;
\`\`\`
1. Data Control Language (DCL) – Manages permissions and access controls. Key commands include:
- `GRANT`: Gives users specific privileges.
\`\`\`sql
GRANT SELECT, INSERT ON students TO user123;
\`\`\`
- `REVOKE`: Removes previously granted privileges.
\`\`\`sql
REVOKE INSERT ON students FROM user123;
\`\`\`
1. Transaction Control Language (TCL) – Manages transaction processing within a database. Commands include:
- `COMMIT`: Saves changes made during a transaction.
\`\`\`sql
COMMIT;
\`\`\`
- `ROLLBACK`: Undoes changes made during a transaction.
\`\`\`sql
ROLLBACK;
\`\`\`
SQL’s power lies in its ability to express complex queries and updates in a relatively simple and highly readable format. Over time, these basics have been expanded upon with additional features like window functions, common table expressions, and advanced join operations, making SQL an extraordinarily versatile tool for interacting with relational databases.
These sources provide both historical context and practical insight into SQL as the primary query language used for interacting with relational databases.
