mysql - 根据行数插入查询

Question

我想要一个查询来将一行插入到表中，我知道这很简单，但情况是该表不应超过 5 行。如果表超过五行，我需要删除旧行（或替换为新行）（基于插入时间戳）然后我需要插入一个新行。如果行数少于 5，那么我可以直接插入一行。

请分享我的查询。

Answer 1

这样的事情怎么样。

declare @count int

SELECT @count=COUNT(*)
from EP_ANSWERS

IF (@count<5)
// DO your insert here
ELSE
DELETE FROM TABLE
 WHERE inserttimestamp = (SELECT x.inserttimestamp
                         FROM (SELECT MAX(t.inserttimestamp) AS inserttimestamp
                                 FROM TABLE t) x)
// DO your insert here

Answer 2

如果表不可能有超过 5 行：

DELETE FROM yourtable
 WHERE 5 <= (SELECT COUNT(*) FROM yourtable)
   AND yourtimestamp = (SELECT MIN(yourtimestamp) FROM yourtable)
;

INSERT INTO yourtable ...
;

如果表有可能超过 5 行：

DELETE FROM yourtable
 WHERE 5 <= (SELECT COUNT(*) FROM yourtable)
   AND yourtimestamp NOT IN (SELECT yourtimestamp
                               FROM yourtable
                              ORDER BY yourtimestamp DESC
                              LIMIT 4)
;

INSERT INTO yourtable ...
;

Answer 3

听起来你想在表上放置一个触发器来维护这个规则，在 MySQL 中这样的东西应该可以工作

CREATE TRIGGER trg__my_table__limit_rows
BEFORE INSERT
ON my_table
FOR EACH ROW 
BEGIN
    IF ((SELECT COUNT(1) FROM my_table) = 5)
    BEGIN            
        DELETE FROM my_table
        WHERE id = (SELECT MIN(id) FROM my_table)  -- change this to fit your logic for which record should be removed
    END
END

Answer 4

这里的一些代码是伪代码（您没有编写架构），但我写了您需要完成自己的代码的地方。

DECLARE @NumberOfRowsToInsert INT = -- select from the data you want to insert
DECLARE @MaxNumberOfRows INT = 5
DECLARE @NumberOfExistingRows INT

DECLARE @Query VARCHAR(MAX) = 'SELECT TOP @rows id FROM SomeTable ORDER BY createdDate ASC'

SELECT @NumberOfExistingRows = COUNT(*)
FROM SomeTable

SET @Query = REPLACE(@Query,'@rows',
 CAST(@NumberOfRowsToInsert - (@MaxNumberOfRows - @NumberOfExistingRows))) AS VARCHAR(1))

CREATE TABLE #IdsToDelete(id INT PRIMARY KEY)

INSERT INTO #IdsToDelete
EXEC(@Query)

DELETE FROM SomeTable
WHERE id IN (SELECT * FROM #IdsToDelete)

-- insert here..