By the end of this page you can predict, before you run it, how many rows any join will return. You will know all six join types and what each keeps. You will know the two ways a join goes wrong without ever raising an error, and the two counts that catch each one. That is the whole subject, and it is about a forty-minute read.
Here is what to actually do with it. Go through once end to end for the shape. Then, on the next join you write, run the two counts before you trust any number that comes out of it. Two extra queries, ten seconds, and they catch the class of mistake that ends up in a report.
The short version: a join can drop rows that found no match, and duplicate rows that found several. So count your rows before and after, every time.
One idea decides everything else here, so it gets the picture. A join does not add columns to your table. It builds a new table, and you have to work out its row count yourself.
The original carries a diagram here. In words: Three games listed on the left are matched to review rows listed in the middle. The first game, Parabox, connects by a single line to one review of 12 hours, and appears once in the result column on the right. The second game, Maj'Eyal, connects by three lines to three separate reviews of 174, 96 and 310 hours, and appears three times in the result column, its name repeated on each row. The third game, Until Then, has a line that ends in a cross because no review matches it, and it does not appear in the result column at all. The result column therefore holds four rows, while neither of the two original tables had four rows.
