SQL Basics for Beginners

Hello everyone Let's take a brief look at the basics of SQL. Let's find out what SQL is, what commands, methods and features there are.

Let's start!

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Basic terminology

Database - is some organized set of information.

Relational database - is a database built on a relational data model (according to the mathematical theory of relations).

Database management system (DBMS) - is a set of programs that allows you to manage the creation and use of a database.

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SQL Language

SQL (structured query language) - is a programming language designed to work with relational databases.

The peculiarity of SQL - is a declarative programming language, we describe what we want to get, not how

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Types of SQL statements

Groups of SQL statements:

• Operators for working with database objects.
• Data manipulation operators.
• Transaction Management Commands
• Data protection and management operators.

We will consider only data manipulation operators, because it is with them that you will most often meet in the course of your work.

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SELECT request. Basic syntax

• Minimum query:

SELECT * FROM table WHERE condition;

• "Separate" and "order by" operators:

SELECT DISTINCT f1, f2 FROM table
WHERE condition ORDER BY f2 DESC;

• Graph operator:

SELECT QUANTITY (*) FROM table;

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Subqueries

Example: print the count of different values in the "f1" field of the table.

SELECT COUNT(*) FROM (SELECT DISTINCT f1 FROM table WHERE condition);

The subquery is written in parentheses, the result of its work is a table.

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The exists operator

A quick way to check if a query returns at least one value.

SELECT smth FROM table 
WHERE EXISTS (SELECT * FROM table2 WHERE table2.field = table.field1);

Sometimes it is convenient to use aliases:

SELECT field1 as f1, field2 as f2
FROM table AS first_table
WHERE EXISTS
(SELECT * FROM table2 AS second_table
WHERE second_table.field = first_table.f1);

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Group by operator

SELECT field, COUNT(*) FROM table 
WHERE condition GROUP BY field HAVING having_condition;

In the HAVING operator, unlike where, aggregating functions can be used.

Some aggregating functions:

• SUM
• COUNT
• MIN
• MAX

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Merging tables

SELECT column_name(s) FROM table1
UNION (UNION ALL)
SELECT column_name(s) FROM table2

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Keys

Types of key relationships:

• 1 to 1
• 1 to many
• many to 1

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Many to many relationship

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JOIN operator

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Changing the contents of tables

• Adding an element:

INSERT INTO table_name values (v1, v2, …);
INSERT INTO table_name (col1, col2) VALUES (v1, v2);

• Updating an element:

UPDATE table_name SET col1 = val1, col2 = val2 WHERE condition;

• Deleting an element:

DELETE FROM table_name WHERE condition;

I hope my efforts will help someone to deal with such a topic as SQL.

I wish you good luck!

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Look into the following series:

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Comments

[Vadim Kolobanov]

Of course! No problem! I will be very glad if the material helps as many people as possible! Just don't forget to leave a link to the source. This will help attract even more

[arsalan0974]

Thank you sir

[Vadim Kolobanov]

Always happy to help