如果table2有更新，则更新table1中的记录（MySQL）

Question

In MySQL, I have 2 tables (currently MyISAM). Let's call them table1 and table2.

table1 has:

id
passwd
full_name
email

table2 has:

user_id
book (the book's title)

Basically, a user can rent books, and a row is created in table2, with records about the user_id, and the book title the user rented.

In table1, I would like to add a column called 'books_rented'.

In PHP, I can calculate all the rows that x user_id has, and it will return how many books that person rented. However, I would like to know if it's possible to do that within MySQL itself, it would seem more optimal to me.

PS.: I am giving books as an example as I thought it would make it simpler, but the tables are actually employer/employee relational. If an employee deletes his account, then it won't really be optimal doing it with PHP as it would need to wait until the employer logs in again to update this. I can't really do this with PHP unless I run a cron job which I don't really like.

Answer 1

what is relation between that 2 tables, i assume that table1.id with table2.id.

update table1 set books_rented=(select count(*) from table2 where user_id=id)

Please try this at your dev machine first.

Answer 2

I think adding the books_rented column is necessary. You can just simply query it to get the quantity of books that a specific user has rented.

SELECT T1.id
    , T1.passwd
    , T1.full_name
    , T1.email
    , IFNULL(COUNT(T2.book), 0) AS books_rented
    FROM table1 T1
    LEFT JOIN table2 T2 ON T2.user_id = T1.id
    GROUP BY T1.id

I suggest that you work with your design, since there are instances that you will be:

• renting the same book (will that add to the count of books rented?)
• are you counting only the books that are currently being borrowed? Or all the books that have been rented before are included?

Since users and books are many to many relationship , you need to add a JUNCTION TABLE . So you will end up with: users , books and userbooks .

Answer 3

Triggers are exactly what I was looking for. Great tutorial about it .