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.
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.
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
- 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.
- 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;
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.
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);
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:
SELECT column_name(s) FROM table1 UNION (UNION ALL) SELECT column_name(s) FROM table2
Types of key relationships:
- 1 to 1
- 1 to many
- many to 1
- 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.
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