The SQL exam made mistake on join, I figure that I didn’t fully understand left and right joins. I study it and recorded it to prevent mistakes:
left join: all the data in the left table will be show on the results, and the data which is not in the right table is showed as NULL
right join: all the data in the right table will be show on the results, and the data which is not in the left table is showed as NULL
iner join: the data in both tables will be show on the results, and data that is not in one table is not showed
join: same as above
outer join: both tables are showed, and all the data that is not in the table is NULL
cross join: generate the Cartesian product of the data in two tables. tremendous result
