SQL自我计算字段

Question

I have two tables which are connected by m2m relationship.

CREATE TABLE words 
    (
    id INT PRIMARY KEY, 
    word VARCHAR(100) UNIQUE, 
    counter INT
    )

CREATE TABLE urls 
    (
    id INT PRIMARY KEY, 
    url VARCHAR(100) UNIQUE
    )

CREATE TABLE urls_words 
    (
    url_id INT NOT NULL REFERENCES urls(id), 
    word_id INT NOT NULL REFERENCES words(id)
    )

and i have counter field in words table. How can i automize proccess of updating counter field which is responsible for calculating how much rows stored in urls_words with particular word.

Answer 1

I would investigate why you want to store this value. There may be good reasons, but triggers complicate databases.

If this is a "load-then-query" database, then you can update the count when you load data -- presumably at some frequency such as once a day or once a week. You don't need to worry about triggers.

If this is a transactional database, then triggers would be needed and these add complexity to the processing. They also lock tables when you might not want them locked.

An alternative is to have an index on urls_words(word_id, url_id) . This would greatly speed the calculation of the count when you need it. It also does not require triggers or locks on multiple table during an update.

Answer 2

在urls_words表上创建一个触发器，每次进行更改（即更新，插入，删除）时，都会更新单词表上的计数器列。