sql - 尝试更新 AFTER UPDATE 触发器上的只读列错误

Question

我正在尝试在 firebird 2.5 上创建一个触发器，以便在更新另一列时更新一列。我为我的尝试创建了一个简单的示例表。

create tablea (estado char(1), fl_previa_laudo char(1));

我的触发器是这样的:

create trigger ATUALIZA_PREVIA_AI for TABLEA
active after update position 0
as
begin

if( old.estado in ('3', '4', '7', '8') ) then
    new.fl_previa_laudo = 'T';
else 
    new.fl_previa_laudo = 'F';
end;

当我运行触发器时，它给出了一个错误:

can't format message 13:849 -- message file C:\Windows\firebird.msg not found. attempted update of read-only column.

Answer 1

您正在尝试修改 AFTER UPDATE 触发器中的列，但这是不可能的。如果要修改值，则需要使用 BEFORE UPDATE 触发器。参见Firebird documentation on triggers ，具体来说(强调我的):

The NEW and OLD variables are subject to some rules:

• In all triggers, the OLD value is read-only
• In BEFORE UPDATE and BEFORE INSERT code, the NEW value is read/write, unless it is a COMPUTED BY column
• In INSERT triggers, references to the OLD variables are invalid and will throw an exception
• In DELETE triggers, references to the NEW variables are invalid and will throw an exception
• In all AFTER trigger code, the NEW variables are read-only

换句话说，before update 触发器允许您在持久化行之前修改值，而 after update 触发器在行持久化后触发，从而允许您可以看到最终的值。