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

No, it's a join. 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's 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 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.