What is the difference between the relationships between tables?

Question

I know there are three types of relationships between data from different tables:

• 1 : 1 (one to one);
• 1 : N (one for several) ;
• N : N (several for several);

But what would be the difference between relationship types and their use in a relational database?

Answer 1

This is called cardinality.

In a way what is in the question practically already says what is each one.

1:1 means that for each row of a table will be referenced in one row in the other table and vice versa.

1:N means that for each row in a table several rows will be referenced in the other table which in turn each row can only have one row referenced in the first table. In general it can be from 0 to N lines.

M:N means that for each row in a table several rows will be referenced in the other table and the latter may have in each of its rows several references to the first table. In general it can be from 0 to N lines.

I don’t like that N:N. This implies that they both have the same number, which is not usually true. I know a lot of people use it like this, but I prefer M:N.

It is also used, 0.. 1, 0.. * or 1.. * to indicate more precisely how many lines can be referenced at least (before points) and at most (after points) indicating whether none can be (0), must be one (1) or may be several (*).