select * from a left join b on a.a=b.b

The query select * from a left join b on a.a = b.b performs a left outer join of tables a and b on the condition that the columns a.a and b.b are equal.

This means that all rows from table a will be included in the result. For those rows from a that have matching values in b based on the condition a.a = b.b, the corresponding data from b will be added. If there are no matches, the fields from b will be filled with NULL.

Example:

a.a	a.other	b.b	b.other
1	foo	1	bar
2	baz	NULL	NULL

Here, the row with a.a = 2 has no match in b, but it is still included in the result.