查询SQL多表

Question

I have a sql database with the following tables

Book Table

CREATE TABLE IF NOT EXISTS `books` (
`book_id` varchar(8) NOT NULL DEFAULT '',
`book_title` varchar(100) DEFAULT NULL,
`author1` varchar(20) NOT NULL,
`author2` varchar(20) DEFAULT NULL,
`publisher` varchar(20) NOT NULL,
`pub_year` year(4) NOT NULL,
`mod_id` varchar(8) NOT NULL,
`courseID` varchar(8) NOT NULL,
PRIMARY KEY (`book_id`),
KEY `id` (`book_id`),
KEY `book_id` (`book_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

Courses Table

CREATE TABLE IF NOT EXISTS `courses` (
`courseID` varchar(8) NOT NULL,
`course_title` varchar(255) CHARACTER SET ascii NOT NULL,
`Entry_Year` int(1) NOT NULL,
`Duration` int(1) NOT NULL,
PRIMARY KEY (`courseID`),
KEY `courseID` (`courseID`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

Modules Table

CREATE TABLE IF NOT EXISTS `modules` (
`mod_id` varchar(8) NOT NULL,
`mod_title` varchar(255) NOT NULL,
`description` varchar(255) NOT NULL,
 PRIMARY KEY (`mod_id`),
 KEY `mod_title` (`mod_title`)
 ) ENGINE=InnoDB DEFAULT CHARSET=latin1;

Module Course Table

CREATE TABLE IF NOT EXISTS `mod_course` (
`ID` int(11) NOT NULL AUTO_INCREMENT,
`module` varchar(8) NOT NULL,
`course` varchar(8) NOT NULL,
PRIMARY KEY (`ID`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 AUTO_INCREMENT=31 ;

I would like to query the database to show all book details of a course. A course has many modules and modules have many books. I have tried the following query but I think there is a problem with my tables as well. (FYI 'BIT' is the id of a course that is in the module course table)

SELECT b.book_id, b.book_title, b.author1, b.author2, b.publisher, b.pub_year, b.mod_id, mc.course
                        FROM books b
                         JOIN mod_course mc
                        WHERE mc.course = 'BIT'

Answer 1

After the keyword Join it should be ON not WHERE

SELECT B.*, MC.COURSE
FROM MODULECOURSE AS MC
INNER JOIN COURSES AS C
ON C.COURSE_TITLE = MC.COURSE
INNER JOIN BOOKS AS B
ON B.COURSEID= C.COURSEID

Answer 2

based on the stated relationship course->module->books, you should drop the courseid column from the books table. the modid that you have is all you need.

the problem with your query is the missing join condition. below the JOIN mod_course mc line you should add ON mc.module = b.mod_id

Answer 3

I have had a deeper look at this and offer the following suggestion which includes changes to the data model also. This solution will also allow you to have the same book used for several modules (if required in the future). Also note the new table mod_books The full code for this (including the query) is below... hope its ok

CREATE TABLE IF NOT EXISTS books
(book_id varchar(8) NOT NULL DEFAULT '',
 book_title varchar(100) DEFAULT NULL,
 author1 varchar(20) NOT NULL,
 author2 varchar(20) DEFAULT NULL,
 publisher varchar(20) NOT NULL,
 pub_year year(4) NOT NULL,
PRIMARY KEY (book_id)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

CREATE TABLE IF NOT EXISTS courses
(course_ID varchar(8) NOT NULL,
 course_title varchar(255) CHARACTER SET ascii NOT NULL,
 Entry_Year int(1) NOT NULL,
 Duration int(1) NOT NULL,
 PRIMARY KEY (course_ID)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

CREATE TABLE IF NOT EXISTS modules
(mod_id varchar(8) NOT NULL,
 mod_title varchar(255) NOT NULL,
 description varchar(255) NOT NULL,
 PRIMARY KEY (mod_id)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

CREATE TABLE IF NOT EXISTS mod_books
(mod_id varchar(8) NOT NULL,
 book_id varchar(8) NOT NULL,
 PRIMARY KEY (mod_id,book_id),
 FOREIGN KEY (mod_id) REFERENCES modules (mod_id),
 FOREIGN KEY (book_id) REFERENCES books (book_id)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

CREATE TABLE IF NOT EXISTS mod_course
(ID int(11) NOT NULL AUTO_INCREMENT,
 mod_id varchar(8) NOT NULL,
 course_ID varchar(8) NOT NULL,
 PRIMARY KEY (ID),
FOREIGN KEY (mod_id) REFERENCES modules (mod_id),
FOREIGN KEY (course_ID) REFERENCES courses (course_ID)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 AUTO_INCREMENT=31 ;

and the query would be

SELECT  b.*
FROM    mod_course mc
INNER JOIN mod_books mb
    ON mb.mod_id = mc.mod_id
    INNER JOIN books b
    ON b.book_id = mb.book_id
WHERE   mc.course_id = 'BIT'