mysql通过触发器设置最大行数 [英] mysql setting max number of rows with a trigger

问题描述

我想创建一个最多包含5行的表.因此，如果我添加新行并且表已经有5行，则触发器应删除第一行并添加新行.

I want to create a table that should have max 5 rows. So if I add a new row and the table already have 5 rows, the trigger should delete the first row and add the new row.

例如:

 my_table

 id | data
 ---------
 1  | a
 ---------
 2  | b
 ---------
 3  | c
 ---------
 4  | d

更改为

 my_table

 id | data
 ---------
 2  | b
 ---------
 3  | c
 ---------
 4  | d
 ---------
 5  | e

推荐答案

您可以使用触发器来做到这一点:

You can do that with a trigger:

CREATE TRIGGER handleFiveRows BEFORE INSERT ON tableName
FOR EACH ROW
BEGIN
    IF (SELECT COUNT(*) FROM tableName) = 5 THEN
        DELETE FROM tableName
        ORDER BY id 
        LIMIT 1;
    END IF;    
END;

每当您有5行时，这将删除ID最小的行，并且在插入后您将再次以5行结束.

Whenever you have 5 rows, this will delete the row with the smallest id and after your insert you will end up with 5 rows again.

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