没有WHERE子句的MySQL LEFT JOIN不返回空行

Question

EDIT: Here's my table:

CREATE TABLE IF NOT EXISTS `punch` (
  `name` varchar(50) NOT NULL,
  `date` varchar(50) NOT NULL,
  `duration` int(11) NOT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

INSERT INTO `punch` (`name`, `date`, `duration`) VALUES
('foo', '1', 2),
('bar', '1', 3),
('bar', '2', 5),
('foo', '3', 6),
('foo', '4', 8),
('bar', '4', 9);

I have table with values below:

SELECT * FROM `punch` P1 WHERE P1.date BETWEEN 1 AND 3 ORDER BY P1.name , date;

result:

name    date    duration
bar        1    3
bar        2    5
foo        1    2
foo        3    6

I want to make a report for date 1 until 3 like this:

name    date    duration
bar        1    3
bar        2    5
bar        3    null
foo        1    2
foo        2    null
foo        3    6

I tried this query (note the commented WHERE):

SELECT * FROM (
    SELECT DISTINCT date FROM punch WHERE date BETWEEN 1 AND 3
) P1
LEFT JOIN (
    SELECT * FROM punch -- WHERE name = 'bar'
) P2 ON P1.date=P2.date

ORDER BY P2.name, P1.date

I got result:

date    name    date    duration
1       bar     1       3
2       bar     2       5
1       foo     1       2
3       foo     3       6

I was expecting something like:

date    name    date    duration
2       NULL    NULL    NULL
3       NULL    NULL    NULL
1       bar     1       3
2       bar     2       5
1       foo     1       2
3       foo     3       6

Now, when I remove the commented WHERE, I got result:

date    name    date    duration
3       NULL    NULL    NULL
1       bar     1       3
2       bar     2       5

My question is, why LEFT JOIN above, behaves like INNER JOIN when there's no WHERE clause?

And what is the correct query for my expected report above?

Thanks

Answer 1

Hi something like this gonna work:)

 SELECT DISTINCT 
    P1.name, 
    P2.date,
    (SELECT PP.duration 
     FROM punch PP 
       WHERE P1.name = PP.name
         AND P2.date = PP.date ) AS duration
    FROM
      (SELECT DISTINCT name FROM  `punch`) P1,
      (SELECT DISTINCT date FROM punch)P2
        WHERE P2.date BETWEEN 1 AND 3
        ORDER BY P1.name , P2.date

Results:

NAME    DATE    DURATION
bar        1    3
bar        2    5
bar        3    (null)
foo        1    2
foo        2    (null)
foo        3    6

SQLFIDDLE example

Answer 2

Try the following:

SELECT * FROM (
    SELECT DISTINCT date FROM punch WHERE date BETWEEN 1 AND 3
) P1
LEFT OUTER JOIN (
    SELECT * FROM punch  WHERE name = 'bar'
) P2 ON P1.date=P2.date

An OUTER JOIN should answer your issue. It includes non-matching elements from the second table.

Look here for more info about how it works: http://infogoal.com/sql/sql-outer-join.htm

Answer 3

Create additional table with day numbers and join punch table with this table.

CREATE TABLE days(day_num INT(11));

INSERT INTO days(day_num) VALUES (1),(2),(3),(4),(5);

Add more records if you need

---

Result query -

SELECT p.* FROM days d
LEFT JOIN punch p
  ON p.date = d.day_num
WHERE
  d.day_num BETWEEN 1 AND 3
ORDER BY
  p.name, p.date;