[英]MYSQL delete all rows, but the same query on select return only 3 rows
我無法理解為什么select和delete的相同查詢具有不同的行為。
我需要刪除除5個最新行之外的所有行。
我知道我的這個任務的解決方案並不好,但我的問題是為什么MySQL沒有刪除相同的行,返回select為同一個查詢子句
看代碼
drop table if exists tbl;
create table tbl
(
id serial,
cal date COMMENT 'some column',
created_at datetime default NOW()
);
insert into tbl
values
(default, '2018-07-15', '2018-07-15 12:00'),
(default, '2018-07-16', '2018-07-16 12:00'),
(default, '2018-07-17', '2018-07-17 12:00'),
(default, '2018-07-18', '2018-07-18 12:00'),
(default, '2018-08-01', '2018-08-01 12:00'),
(default, '2018-08-04', '2018-08-04 12:00'),
(default, '2018-08-16', '2018-08-16 12:00'),
(default, '2018-08-17', '2018-08-17 12:00');
select *
from tbl;
# +----+------------+---------------------+
# | id | cal | created_at |
# +----+------------+---------------------+
# | 1 | 2018-07-15 | 2018-07-15 12:00:00 |
# | 2 | 2018-07-16 | 2018-07-16 12:00:00 |
# | 3 | 2018-07-17 | 2018-07-17 12:00:00 |
# | 4 | 2018-07-18 | 2018-07-18 12:00:00 |
# | 5 | 2018-08-01 | 2018-08-01 12:00:00 |
# | 6 | 2018-08-04 | 2018-08-04 12:00:00 |
# | 7 | 2018-08-16 | 2018-08-16 12:00:00 |
# | 8 | 2018-08-17 | 2018-08-17 12:00:00 |
# +----+------------+---------------------+
現在我需要刪除ID為1,2,3的行
SET @row_number = 0;
select *
from tbl
where tbl.id in (
select T.id
from (SELECT (@row_number := @row_number + 1) as num, tbl.id
from tbl
order by created_at desc
) as T
where T.num > 5);
# +----+------------+---------------------+
# | id | cal | created_at |
# +----+------------+---------------------+
# | 3 | 2018-07-17 | 2018-07-17 12:00:00 |
# | 2 | 2018-07-16 | 2018-07-16 12:00:00 |
# | 1 | 2018-07-15 | 2018-07-15 12:00:00 |
# +----+------------+---------------------+
現在我使用刪除操作
SET @row_number = 0;
delete
from tbl
where tbl.id in (
select T.id
from (SELECT (@row_number := @row_number + 1) as num, tbl.id
from tbl
order by created_at desc
) as T
where T.num > 5);
select * from tbl; # <-- result empty
# +----+-----+------------+
# | id | cal | created_at |
# +----+-----+------------+
我哭了;
我們可以嘗試在這里執行刪除限制連接:
DELETE t1
FROM tbl t1
LEFT JOIN
(
SELECT id
FROM tbl
ORDER BY created_at DESC
LIMIT 5
) t2
ON t1.id = t2.id
WHERE
t2.id IS NULL;
這種反連接背后的想法是,我們將刪除任何與前五個記錄之一不匹配的記錄,這些記錄按created_at
列的順序遞減。
請注意,我們不能WHERE IN
此處使用WHERE IN
查詢,因為MySQL將返回此版本尚不支持LIMIT
的可怕錯誤消息。
使用LIMIT
和OFFSET
獲取要刪除的最高ID:
set @last_id_to_delete = (
select id
from tbl
order by id desc
limit 1
offset 5
);
然后刪除ID等於或小於obove值的所有行:
delete tbl
from tbl
where id <= @last_id_to_delete;
您可以將兩個查詢合並為一個。 使用WHERE子句中的子查詢:
delete tbl
from tbl
where id <= (select id from(
select id
from tbl
order by id desc
limit 1
offset 5
)x);
(請注意,您需要將子查詢結果包裝到派生表中,以避免錯誤:“您無法在FROM子句中為更新指定目標表'tbl'。”
或者通過加入單行子查詢:
delete t
from tbl t
join (
select id as last_id_to_delete
from tbl
order by id desc
limit 1
offset 5
) x on t.id <= x.last_id_to_delete;
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