The first chapter is ready

What is a connection:

An equivalent connection is formed by retrieving all data from two detached data sources and merging them into a large table.

An inner join is connected to an internal column of two tables.

An outer join is connected to an external column of two tables.

Left join is connected to the left column of two tables.

The right connection connects to the right connection of the two tables.

SQL and a very clear question of the relationship:

The exact meaning of database, relationship and tuple;

The difference between the relationship value and the relationship variable;

The relevance of predicates and propositions;

The importance of attribute names;

The key role of integrity constraints;

The principle of information and its importance;

You may realize that I used the formal term "relationship (relation)", "tuple" and "attribute (Attribute)"; SQL certainly does not use these Terms and instead is more "in friendly" terms: "table", "Row", "column". The truth is that relationships are not tables, tuples are not rows, and attributes are not columns; only when we all understand that the "user-friendly" term is just an approximation of the truth and cannot grasp the nature of the fact,

It can be accepted. In other words: If you do understand the real situation, it is a good idea to use the "user-friendly" terminology reasonably, but if you learn and understand the real situation as the first priority, you need to use

A formal term.

The original relational model has 3 main components: "Structure (structure)", "Integrity (Integrity)", "Operation (mainpulation)"

The first chapter is ready