在条件下使用 DATE() 时，LEFT JOIN 像 INNER 一样工作

Question

I have the following sample data, query and result:

DROP TABLE IF EXISTS calendar;

CREATE TABLE `calendar`  (
  `day_date` date NOT NULL,
  PRIMARY KEY (`day_date`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Compact;

DROP TABLE IF EXISTS finance;

CREATE TABLE `finance`  (
  `finance_id` int(11) NOT NULL,
  `timestamp` datetime(0) NOT NULL,
  `debit` int(11) NULL DEFAULT NULL,
  `credit` int(11) NULL DEFAULT NULL,
  PRIMARY KEY (`finance_id`) USING BTREE,
  UNIQUE INDEX `finance_id`(`finance_id`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_bin ROW_FORMAT = Dynamic;


insert into `calendar` values 
('2020-01-01'),
('2020-01-02'),
('2020-01-03'),
('2020-01-04'),
('2020-01-05');

insert into finance values
(1, '2020-01-02 00:00:00', 100, 0),
(2, '2020-01-04 9:00:00', 200, 0);


select day_date , finance.`timestamp`, debit 
from calendar
left join finance on calendar.day_date = date(finance.`timestamp`);

+------------+---------------------+-------+
| day_date   | timestamp           | debit |
+------------+---------------------+-------+
| 2020-01-02 | 2020-01-02 00:00:00 |   100 |
| 2020-01-04 | 2020-01-04 09:00:00 |   200 |
| 2020-01-01 | NULL                |  NULL |
| 2020-01-03 | NULL                |  NULL |
| 2020-01-05 | NULL                |  NULL |
+------------+---------------------+-------+

Where day_date is date, while timestamp is datetime.

The problem is that when I use date() in the on clause, the query only returns matching records in both tables, while it should get ALL records from calendar, along with any matching records from finance.

However, when I remove date() function from the on clause, it works as it should. Any idea why it's behaving like this?

Update: I tried the query in dbfiddle here: https://www.db-fiddle.com/f/bVvbUy1rirBp5tD9Fq9J8X/0 and it seems it's working fine there. The problem only appears in my environment (MySql v5.7).

Answer 1

Your first datetime for the date field '2020-01-02' is '2020-01-02 00:00:00'

When you compapre a date with a datetime you get the comparison of '2020-01-02 00:00:00' vs '2020-01-02 00:00:00' and that is equal

But for the 4th

you have '2020-01-04 9:00:00' but the date to compare is '2020-01-04 00:00:00', because a date adds the 00:00:00 automatically for comparison and is not equal

but when you make a date('2020-01-04 9:00:00') you receive '2020-01-04' and that is again equal to the date field '2020-01-04'

So the difference are the hours that are compare in datetime they are relevant in date not