Setting a field equal to the row count of another table

[Redline]

Is there a way, in MySQL, to set a field in table "A" equal to a row count of another table where id = 'x'?

What I'm doing is building my own blog, more accurately the commenting system. I have a field in the table containing the blog information, which includes a field "num_comments". I'd like to be able to post comments without having to update both the comments table, and the blog table. I'm wondering if there is a specific function within MySQL that will allow me to do this?

While I'm here, how would I go about incrementing an int field without querying for the value first?

[l33t_1-1axx0r]

Maybe I'm a little confused as to what you're asking, but it seems that the number of comments and the blog information is seperate... so you'd never need to update two tables at once. Instead, maybe have a field in the comments table that holds the ID of the blog it corresponds to...

A little more detail on how you're structuring may help.

As for the autoincrementing, just add NOT NULL AUTO_INCREMENT to the column when you create the tables.

e.g.

CREATE TABLE foo (
     id INT NOT NULL AUTO_INCREMENT,
     bar CHAR(30),
     PRIMARY KEY (id)
);

[redhead]

He's talking about the FOREIGN activation ie:

CREATE TABLE foo (
    id INT NOT NULL auto_increment,
    UNIQUE id (id),
    PRIMARY KEY (id)
);
CREATE TABLE bar (
    id INT NOT NULL auto_increment,
    foo_id INT NOT NULL,
    UNIQUE id (id),
    PRIMARY KEY (id)
    FOREIGN KEY (foo_id) REFERENCES foo
);