User-defined variable in an INSERT query for MySQL

Question

I need to use user-defined variable in an INSERT query for MySQL, see an example bellow:

INSERT INTO `posts`(`id`) VALUES(NULL);
SET @last_insert_id = LAST_INSERT_ID();
INSERT INTO `comments`(`id`, `post_id`) VALUES(NULL, "@last_insert_id");

This example doesn't work and inserted 0. What am I doing wrong?

Answer 1

There is no need to store it in a variable. You can just call LAST_INSERT_ID() inside the following INSERT statement.

INSERT INTO `comments`(`id`, `post_id`) VALUES (NULL, LAST_INSERT_ID());

... unless you have multiple inserts to perform using that id.

In that case, the proper syntax for using the variable is to do so without quotes:

INSERT INTO `posts`(`id`) VALUES (NULL);
SET @last_insert_id = LAST_INSERT_ID();
/* Several new entries using the same @last_insert_id */
INSERT INTO `comments`(`id`, `post_id`) VALUES (NULL, @last_insert_id);
INSERT INTO `comments`(`id`, `post_id`) VALUES (NULL, @last_insert_id);
INSERT INTO `comments`(`id`, `post_id`) VALUES (NULL, @last_insert_id);
INSERT INTO `comments`(`id`, `post_id`) VALUES (NULL, @last_insert_id);
INSERT INTO `comments`(`id`, `post_id`) VALUES (NULL, @last_insert_id);