插入触发器之前的mysql错误代码1364：字段没有默认值和错误代码1442。无法更新存储的函数/触发器中的表

Question

I have a hierarchical table (left/right values to determine the hierarchy) in my database and I would like to add a "before insert" trigger. One of the problems with large hierarchical tables of this sort is that it can be hard to figure out what has to change when you insert a new item into the hierarchy. So I wanted to automate this in my trigger.

So, my table looks like this...

    name varchar(25) not null,
    leftValue int not null,
    rightValue int not null

Some data...

    'DEFAULT',           1, 10
    'FirstChild',        2, 3
    'SecondChild',       4, 7
    'SecondChildsChild', 5, 6
    'ThirdChild',        8, 9

Now I want to add a 'FirstChildsChild'. This will affect every row except default. I would like the user to only have to do something simple like this...

    SET parent = 3;  -- The left value of FirstChild
    insert into myTable (name, leftValue) VALUES ('FirstChildsChild', parent)

My new table (after properly executing the trigger that I am having trouble with) should look like this...

    'DEFAULT',           1, 12
    'FirstChild',        2, 5
    'FirstChildsChild',  3, 4
    'SecondChild',       6, 9
    'SecondChildsChild', 7, 8
    'ThirdChild',        10, 11

So I wrote the following trigger...

    CREATE TRIGGER `myTrigger` BEFORE INSERT ON `myTable` FOR EACH ROW
    begin
        SET new.rightValue = new.leftValue + 1;

        UPDATE MissionFactors SET
            leftValue = leftValue + 2
        WHERE
            leftValue >= new.leftValue;

        UPDATE MissionFactors SET
            rightValue = rightValue + 2
        WHERE 
            rightValue >= new.leftValue;
    END

FINALLY my questions: 1. Since the trigger is a BEFORE insert, why won't it let me leave the right value undefined in my insert statement? The trigger fills in the value.

    Error Code: 1364. Field 'rightValue' doesn't have a default value.

1. If I do pass in a right value...

    Error Code: 1442. Can't update table 'mytable' in stored function/trigger because it is already used by statement which invoked this stored function/trigger. 

I could write a stored procedure to do all of this. But I thought that this type of thing was one of the reasons why triggers were created.

Answer 1

I encountered the same problem.

You have to put the column nullable if you don't want it to appear in your insert's column list even if a trigger sets a default value.

For your second problem, you can't update a table while it's open for inserting. In Oracle there is a similar problem called 'mutating table'. If you want to do this you have to use several triggers including an after statement trigger that update the table.