Category Archives: Без рубрики

FULL OUTER JOIN Explained A FULL OUTER JOIN: Returns all records from both tables. If there’s a match between A and B, it combines them. If there’s no match, you still get the row — with NULL in the missing … Continue reading →

Main Types of JOINs JOIN Type What It Does INNER JOIN Returns rows where there’s a match in both tables. LEFT JOIN Returns all rows from the left table, plus matches from the right (NULL if no match). RIGHT JOIN … Continue reading →

Denormalization is the process of intentionally adding redundancy into a database — basically undoing some of the normalization — in order to improve read performance. Key Idea: In normalization, we split data into multiple related tables to reduce redundancy. In … Continue reading →

Third Normal Form (3NF) Goal: Be in 2NF AND Eliminate transitive dependencies (non-key columns must depend only on the primary key, not on other non-key columns). A table in 2NF but NOT in 3NF: student_id student_name student_zipcode zipcode_city 1 Alice … Continue reading →

What is a Partial Dependency? A partial dependency happens when a column (like student_name) depends on part of a composite key (like student_id), not the whole key (student_id, course). Let’s See the Table Again Imagine this table: student_id course student_name … Continue reading →

First Normal Form (1NF) Goal: Eliminate repeating groups. Make sure each field contains atomic (indivisible) values. A table NOT in 1NF: student_id name courses 1 Alice Math, History 2 Bob Math Problem: courses has multiple values in one column — … Continue reading →

What is Normalization? Normalization is the process of organizing data in a database to reduce redundancy (duplicate data) and improve data integrity. It involves breaking a big, messy table into smaller, more focused tables and linking them through relationships (usually … Continue reading →

INNER JOIN: Returns only the rows that have matching values in both tables. If there’s no match between the two tables, that row is excluded from the result. LEFT JOIN (or LEFT OUTER JOIN): Returns all rows from the left … Continue reading →

When you set up a foreign key with ON DELETE CASCADE, it means: If you delete a row in the parent table (the table with the primary key), all related rows in the child table (the table with the foreign … Continue reading →

A foreign key is a column (or a set of columns) in a table that creates a link between data in two tables. It acts as a reference to a primary key in another table. Key points about foreign keys: … Continue reading →