What happens when performing a LEFT JOIN between two tables, for example, using Euler circles to illustrate the connection?

LEFT JOIN returns all rows from the left table and the matching rows from the right table. If there is no match for a row in the left table, the result will contain NULL values for the right table.

An illustration with Euler circles:

• Imagine two circles, where the left one is the left table, and the right one is the right table.
• LEFT JOIN is the entire left circle plus the intersection with the right.
• That is, all elements of the left circle will be in the result, and elements of the right — only those that intersect with the left.

SQL example:

SELECT * FROM A LEFT JOIN B ON A.id = B.a_id;

If there are records in table A without corresponding records in B, they will still appear in the result, but the fields from B will be NULL.