如何创建一个SQL选择，该选择将创建一个范围的所有数据并计算出现次数

Question

I want to take data in one table that has records recording when events took place on certain dates and create output that has data for the complete date range with the number of events per person per date counted.

For example if I have a Source table containing data like this:

neil 01-05-2015
john 02-05-2015
neil 01-05-2015
neil 02-05-2015
jane 03-05-2015

A a second table containing all dates:

01-05-2015
02-05-2015
03-05-2015
04-05-2015

How can I combine them to create the desired output?

neil 01-05-2015 2
neil 02-05-2015 1
neil 03-05-2015 0
neil 04-05-2015 0
john 01-05-2015 0
john 02-05-2015 1
john 03-05-2015 0
john 04-05-2015 0
jane 01-05-2015 0
jane 02-05-2015 0
jane 03-05-2015 1
jane 04-05-2015 0

I have tried a variety of full joins but nothing seems to get me exactly what I need and I am wondering if it is possible in SQL alone.

Answer 1

Use a CROSS JOIN to first make a full cross-product between the dates and people. Then use a LEFT JOIN to count all the matches.

SELECT s1.name, d.date, IFNULL(COUNT(s2.name), 0)
FROM Dates AS d
CROSS JOIN (SELECT DISTINCT name FROM Source) AS s1
LEFT JOIN Source AS s2 ON s1.name = s2.name AND d.date = s2.date
GROUP BY s1.name, d.date

DEMO