Bit of MySQL_ OR、BIT_ Count function

1. Preface

Recently in to see MySQL Official documents , Find a very clever usage , You can quickly calculate the number of days a user visits the system every month . Now let's analyze the principle .

2. Data preparation

CREATE TABLE data_test (year YEAR, month INT UNSIGNED, day INT UNSIGNED);
INSERT INTO data_test VALUES(2022,1,1),(2022,1,20),(2022,1,30),(2022,2,2),(2022,2,5),(2022,2,5);

From the above table creation statement and data insertion statement 2022 Year of 1 The number of visit days in the month is 3,2 The number of visit days in the month is 2.

According to the general query , You can query according to the following statement ：

with a as (select `year`, `month` from data_test group by `year`, `month`)
select b.`year`, b.`month`, count(distinct day) days
from a
    left join data_test b on a.`month` = b.`month` and a.`year` = b.`year`
group by b.`year`, b.`month`;

+------+-------+------+
| year | month | days |
+------+-------+------+
| 2022 |     1 |    3 |
| 2022 |     2 |    2 |
+------+-------+------+

3. Data query

As usual , We need to check twice to get the result . In fact, we have a simpler way to query , Is the use of MySQL For us BIT_OR、BIT_COUNT function .

SELECT `year`, `month`, BIT_COUNT(BIT_OR(1 << day)) AS days
FROM data_test
GROUP BY `year`, `month`;

+------+-------+------+
| year | month | days |
+------+-------+------+
| 2022 |     1 |    3 |
| 2022 |     2 |    2 |
+------+-------+------+

You can see that the results obtained are consistent with the results of the above query using the conventional method , So what is the principle of these two functions ？ Let's analyze one by one .

• BIT_OR： This function is used to An operation Medium Or operations Of .

• BIT_COUNT： This function is used to calculate 1 The number of .

• 1 << day： Express 1 Moving to the left day bits .

  • 1 << 1 The value is 0000 0000 0000 0000 0000 0000 0000 0010
  • 1 << 20 The value is 0000 0000 0001 0000 0000 0000 0000 0000
  • 1 << 30 The value is 0000 0001 0000 0000 0000 0000 0000 0000

  For this 3 Value for Or operations Words , You can get 0000 0001 0001 0000 0000 0000 0000 0010, take 1 The number of 3

therefore MySQL Get the result through very clever bit operation , It also improves performance .

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