SQLite select from where: основные моменты для эффективного использования

Команда SQLite SELECT используется для выбора данных из таблицы. Она позволяет выбирать данные, которые соответствуют определенным условиям с использованием ключевого слова WHERE.

Вот пример использования SELECT с WHERE:

SELECT * FROM table_name WHERE condition;

Здесь * означает выбор всех столбцов таблицы, table_name - имя таблицы, а condition - условие, которое определяет, какие строки должны быть выбраны.

Пример:

SELECT * FROM customers WHERE age >= 18;

Этот запрос выберет всех клиентов из таблицы "customers", у которых возраст больше или равен 18.

Вы также можете выбирать конкретные столбцы, указав их имена после ключевого слова SELECT:

SELECT column1, column2 FROM table_name WHERE condition;

Пример:

SELECT name, email FROM customers WHERE age >= 18;

Этот запрос выберет только имена и адреса электронной почты клиентов из таблицы "customers", у которых возраст больше или равен 18.

Все строки, которые удовлетворяют условию WHERE, будут возвращены в результате запроса.

SQLite SELECT FROM WHERE: Understanding the Basics

Welcome to the world of SQL and SQLite! In this article, we will dive deep into the topic of SELECT FROM WHERE clause in SQLite. We will cover its syntax, usage, and provide you with code examples to make the learning process interactive and engaging. So, let's get started!

What is the SELECT FROM WHERE Clause?

The SELECT FROM WHERE clause is a fundamental component of SQL queries that allows you to retrieve specific data from a database table based on specified conditions. It is an essential feature for filtering and retrieving relevant data from large datasets.

Syntax of the SELECT FROM WHERE Clause

SELECT column1, column2, ...
FROM table_name
WHERE condition;

In the above syntax:

• SELECT specifies the columns or expressions you want to retrieve from the table. You can select one or more columns by separating them with commas.
• FROM specifies the table or tables from which you want to retrieve the data.
• WHERE specifies the conditions that must be satisfied for a row to be returned in the result set.
• condition represents the filtering logic that defines the conditions for retrieving specific rows. It can include operators such as equal to (=), not equal to (!=), greater than (>), less than (<), and logical operators such as AND and OR.

Code Examples

To better understand the usage of the SELECT FROM WHERE clause, let's consider a fictional database table called students with the following structure and sample data:

CREATE TABLE students (
    id INTEGER PRIMARY KEY,
    name TEXT,
    age INTEGER,
    grade INTEGER
);

INSERT INTO students (name, age, grade)
VALUES ('John Doe', 18, 12),
       ('Jane Smith', 17, 11),
       ('Mike Johnson', 19, 12),
       ('Emma Davis', 18, 11),
       ('Alex Thompson', 17, 10);

Example 1: Retrieving All Columns from the Table

To retrieve all columns from the students table, we can use the following query:

SELECT * FROM students;

This query returns all the rows and columns from the students table.

Example 2: Retrieving Specific Columns with a Condition

Let's say we want to retrieve the names and ages of students whose grade is greater than or equal to 12. We can use the following query:

SELECT name, age FROM students WHERE grade >= 12;

This query will return the names and ages of students whose grade is greater than or equal to 12.

Example 3: Retrieving Rows Based on Multiple Conditions

In this example, let's retrieve the names and grades of students whose age is greater than 17 and grade is equal to 11. The query will be:

SELECT name, grade FROM students WHERE age > 17 AND grade = 11;

This query will return the names and grades of students who satisfy both conditions: age greater than 17 and grade equal to 11.

Example 4: Using the NOT Operator

The NOT operator allows us to negate a condition. Let's retrieve the names of students who are not in grade 10:

SELECT name FROM students WHERE grade != 10;

This query will return the names of students who are in grades other than 10.

Conclusion

Congratulations! You have learned about the SELECT FROM WHERE clause in SQLite. It is a powerful tool that allows you to filter and retrieve specific data from a database table based on your specified conditions. We covered the syntax of the clause and provided you with several code examples for better understanding. Keep practicing and exploring more complex queries to enhance your SQL skills. Happy coding!