mysql setting max number of rows with a trigger

问题:

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.

e.g.:

 my_table

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

change to

 my_table

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

回答1:

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;

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.