PostgreSQL插入触发器以设置值 [英] Postgresql insert trigger to set value

问题描述

假设在Postgresql中，我有一个表 T ，它的列之一是 C1 。

Assume in Postgresql, I have a table T and one of its column is C1.

我想在向表 T 添加新记录时触发一个函数。该函数应检查新记录中列 C1 的值，如果为空/空，则将其值设置为'X'。

I want to trigger a function when a new record is adding to the table T. The function should check the value of column C1 in the new record and if it is null/empty then set its value to 'X'.

这可能吗？

推荐答案

您正确地需要触发器，因为为列设置默认值对您不起作用-默认值仅适用于 null 值，并不能帮助您防止空白值。

You are correct that you need a trigger, because setting a default value for the column won't work for you - default values only work for null values and don't help you in preventing blank values.

在postgres中有一个创建触发器的几个步骤：

In postgres there are a couple of steps to creating a trigger:

步骤1：创建一个返回类型为 trigger 的函数：

Step 1: Create a function that returns type trigger:

CREATE FUNCTION my_trigger_function()
RETURNS trigger AS '
BEGIN
  IF NEW.C1 IS NULL OR NEW.C1 = '''' THEN
    NEW.C1 := ''X'';
  END IF;
  RETURN NEW;
END' LANGUAGE 'plpgsql'

第2步：创建触发的触发器插入之前，它允许您在插入值之前更改值，该值将调用上述函数：

Step 2: Create a trigger that fires before insert, which allows you to change values befre they are inserted, that invokes the above function:

CREATE TRIGGER my_trigger
BEFORE INSERT ON T
FOR EACH ROW
EXECUTE PROCEDURE my_trigger_function()

您已经完成。

请参见上面在SQLFIddle上执行的代码证明其正常工作！

See the above code executing on SQLFIddle demonstrating it working correctly!

您在评论中提到从子查询中检索值'X'。如果是这样，请更改相关行，例如：

You mention in a comment that the value 'X' is retrieved from a subquery. If so, change the relevant line so something like:

NEW.C1 := (select some_column from some_table where some_condition);

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