COUNT on LEFT JOIN

When using COUNT with LEFT JOINs, it is worth noting that is that it does not count NULL values if counting over something different than *.
This is useful if you wish to count how many of the rows fulfill some sort of join clause.

Take the following example.
I’ve created two tables consisting of a unique identifier and a text column. I’ve added test data and for every two row I placed into Tabel_1, I placed one row into Table_2. Then I made a left join query which looks like:

SELECT *
FROM Table_1 AS T
LEFT JOIN Table_2 AS T2 ON T.PK = T2.FK

Which provide an output like this:

LEFT JOIN example for using COUNT

Now to illustrate the difference in count, run a query like this:

SELECT COUNT(*) AS AllRows, COUNT(PK) AS RowsFromLeft, COUNT(FK) AS RowsFromRight
FROM Table_1 AS T
LEFT JOIN Table_2 AS T2 ON T.PK = T2.FK

Which provide the following output:

COUNT result for LEFT JOIN

I’ve seen many more or less creative solutions in my time to how to count over the Right table in a Left Join, because few people seem to read the documentation on something as simple as a COUNT.
However often it is well worth it just to take a peek.