Essential Books

Comments

suggest change

The -- style of comment, which requires a trailing space, differs in behavior from the SQL standard, which does not require the space.

Adding comments

There are three types of comment: 

# This comment continues to the end of line

-- This comment continues to the end of line

/* This is an in-line comment */ 

/*
This is a
multiple-line comment
*/

Example: 

SELECT * FROM t1; -- this is comment

CREATE TABLE stack(
    /* id_user int,
    username varchar(30),
    password varchar(30)
    */
    id int
);

The -- method requires that a space follows the -- before the comment begins, otherwise it will be interpreted as a command and usually cause an error. 

# This comment works
/* This comment works. */
-- This comment does not.

Commenting table definitions 

CREATE TABLE menagerie.bird (
    bird_id INT NOT NULL AUTO_INCREMENT,
    species VARCHAR(300) DEFAULT NULL COMMENT 'You can include genus, but never subspecies.',
    INDEX idx_species (species) COMMENT 'We must search on species often.',
    PRIMARY KEY (bird_id)
) ENGINE=InnoDB COMMENT 'This table was inaugurated on February 10th.';

Using an = after COMMENT is optional. (Official docs)

These comments, unlike the others, are saved with the schema and can be retrieved via SHOW CREATE TABLE or from information_schema.

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Comments:
* Adding comments
* Commenting table definitions
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