SQL Flashcards

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SQL (Structured Query Language) is a fundamental language for managing and manipulating relational databases. Originally developed by IBM in the 1970s, SQL has become the standard language for relational database management systems (RDBMS). It is an essential tool in the data management ecosystem, designed to handle data retrieval, manipulation, and management tasks efficiently. SQL is characterized by its declarative nature and powerful set of commands, enabling effective management of both simple and complex database operations. This language offers advanced concepts such as joins, subqueries, and stored procedures, providing developers and database administrators with tools to create robust, efficient, and scalable database solutions. SQL also supports integration with various programming languages and is regularly updated with new features and improvements, maintaining consistency with modern data management needs and enabling the development of data-driven applications across different platforms and environments.

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What is SQL and what is it used for?

SQL, or Structured Query Language, is the standard programming language used for managing and manipulating databases. SQL allows for the creation, modification, access, and management of data contained in relational databases. The main operations that can be performed using SQL include creating tables (CREATE), inserting data into tables (INSERT), updating data (UPDATE), deleting data (DELETE), and most importantly, retrieving data from the database (SELECT).

SQL is particularly valued for its capabilities in data querying, which allow for effective sorting, filtering, and aggregating of data within complex databases. As a result, this language has found widespread use in various fields requiring efficient management of large data sets, such as data analysis, information systems management, and software development.

SQL supports data management in DBMS (Database Management System) platforms such as Oracle, MySQL, Microsoft SQL Server, and many others, enabling users to store and retrieve data in a structured and efficient manner.

Describe the SELECT, INSERT, UPDATE, DELETE commands.

SQL commands such as **SELECT**, **INSERT**, **UPDATE**, and **DELETE** are basic operations used to manipulate data stored in databases. Here is a brief description of each:

1. **SELECT** - used to retrieve data from a database. It allows selecting one or more columns from one or more tables. It can include various clauses such as WHERE, GROUP BY, HAVING, ORDER BY, which are used for filtering, grouping, and sorting data.

SELECT column1, column2 FROM table WHERE condition;

2. **INSERT** - used to add new records to a table. We can insert values directly by specifying the columns we want to fill and their corresponding values.

INSERT INTO table (column1, column2) VALUES (value1, value2);

3. **UPDATE** - allows modifying existing records in a table. We need to specify the table, columns, and new values, as well as a condition (usually using the WHERE clause), which determines which records should be updated.

UPDATE table SET column1 = value1 WHERE condition;

4. **DELETE** - enables deleting records from a table. Similarly to UPDATE, we typically use the WHERE clause to specify which records should be deleted.

DELETE FROM table WHERE condition;

Managing data using these commands is a fundamental aspect of working with relational databases, and their proper use is crucial for maintaining data integrity and efficiency.

How can tables be joined in SQL?

In SQL, we can join tables using several different types of JOIN so that we can operate on data from multiple tables. Here are examples of JOIN types that can be used to join tables:

1. INNER JOIN - returns records that have matching values in both tables. It is the most commonly used type of join.

2. LEFT JOIN (or LEFT OUTER JOIN) - returns all records from the left table (among those that are joined), and the matching records from the right table. If there are no matching records from the right table, the result for those columns will be NULL.

3. RIGHT JOIN (or RIGHT OUTER JOIN) - returns all records from the right table (among those that are joined), and the matching records from the left table. If there are no matching records from the left table, the result for those columns will be NULL.

4. FULL JOIN (or FULL OUTER JOIN) - returns records when there is a match in either table. If there is no match in one of the tables, the result for that table will be NULL.

Example of using INNER JOIN in SQL:

SELECT A.name, B.address
FROM Employees AS A
INNER JOIN Departments AS B
ON A.department_id = B.id;

In this example, the `Employees` table is joined with the `Departments` table through the `department_id` field, which is a foreign key in the `Employees` table linking records to the `Departments` table. The result of the query will be the names and addresses of those employees who have an assigned department.

What is a Primary Key and a Foreign Key?

**Primary Key** is a column or a set of columns in a database table that uniquely identifies each row in the table. Primary keys are used to ensure data integrity and to create relationships with other tables. Each row must have a unique primary key value, and the value in this column cannot be null (NULL). A primary key can consist of a single column or multiple columns (composite primary key).

**Foreign Key** is a column or a set of columns in one table that refers to the primary key in another table. Foreign keys are used to define and maintain relationships between tables. They allow for data consistency, for example, by preventing the addition of a row to a table if a corresponding row does not exist in the table to which the foreign key refers. Foreign keys also enable querying across multiple tables, which is fundamental to operations in relational databases.

In summary, primary keys and foreign keys are crucial elements in the design of relational databases, allowing for the identification and association of data across multiple tables.