Isn't SQL A left join B, just A?

No, since it's a join, it can produce cardinalities greater than 1 for the rows in A. That is, if there are multiple matching rows from B then a row in A will show up multiple times.

Example:

Table A:

id name
-- -------
 1 Alice
 2 Malcolm
 3 Kelly

Table B:

id_a preferred_food
---- --------------
   1 Pizza
   2 Burger
   2 Steak
   2 Menestroni

Then "A left join B" will give you:

id name    id_a preferred_food
-- ------- ---- --------------
 1 Alice      1 Pizza
 2 Malcolm    2 Burger
 2 Malcolm    2 Steak
 2 Malcolm    2 Menestroni
 3 Kelly   null null

In short:

• All rows from A show up in the left join: even 3 Kelly shows up.
• Columns from B will show up with nulls when there are no matching rows in B: row 3 Kelly has null in the last two columns.
• Rows in A may show up multiple times when they have multiple matches in B: row 2 Malcolm shows up three times.

Your diagram isn't quite a Venn diagram.

The intersection of the two circles represents joined rows (according to your join condition) with data from both table A and table B.

The left crescent (labeled "A") represents rows in table A that do not have any corresponding rows in table B; the right crescent (labeled "B") represents rows in table B that do not have any corresponding rows in table A.

What the top left diagram is supposed to show is that a left join gives you data from both table A and B that can be joined up according to your join condition, plus all rows from table A that have no corresponding match in table B.

With JOINs, you get fields from both tables, not only A. It also multiplies the number of records returned if the relationship between tables is not 1-1.

Basically, the only difference between all those JOIN is the behavior when records in A do not match any records in B and vice versa. Your diagrams only show that behavior.

Diagram top let = keep all the records from A, whether or not they match to anything in B (+ what is not represented: when they match, get the data from B).

A LEFT JOIN B MEANS A + (common ENTRIES OF A WHICH ARE THERE IN B) left join would rather be performed where the table which is left joined has more records in this case A and those records might refer some other columns from B on some common condition. hence, the result A left join B would not mean only A but columns of B added as well in case if required on some common value of A and B.

In that diagram they omit the word 'outer' from that description, but you should read it as LEFT OUTER JOIN.

The table to the "left" of the SQL statement (which is going to be the table after the FROM, and in this case, table A) will have every row returned whether or not there is a corresponding row in Table B that matches the JOIN condition.

This is the difference between an inner join and an outer join. Inner joins only return a row where there is a match on the join condition, whereas, a LEFT OUTER join returns the same rows returned by an INNER join, as well as a ROW for any rows in the LEFT table that don't satisfy the JOIN condition. For those rows in the LEFT table that don't JOIN to one or more rows in Table B, the columns of table B that might be specified in the SELECT will be NULL.

You can see that this property of unmatched LEFT rows having NULL values for table B columns is used in the example directly below to filter out any rows derived from a successful join to table B.