如何进行以下MySQL加入？

Question

Assume I have the following tables:

tableA

a_name | age | country
Jordan | 5 | Germany
Molly | 6 | Spain
Paris | 7 | France
John | 7 | Saudi Arabia
John | 5 | Saudi Arabia
John | 6 | Spain

tableB

id (auto increment primary key) | age | country
1 | 5 | Germany
2 | 6 | Spain
3 | 7 | France
4 | 7 | Spain
5 | 8 | France
6 | 9 | France
7 | 2 | Mexico
8 | 7 | Saudi Arabia
9 | 5 | Saudi Arabia

I want to be able to do some kind of select where I am able to get:

tableA

a_name | age | country | id
Jordan | 5 | Germany | 1
Molly | 6 | Spain | 2
Paris | 7 | France | 3
John | 7 | Saudi Arabia | 8
John | 5 | Saudi Arabia | 5
John | 6 | Spain | 4

Iteratively I fill in what is in the "id" field by looking up what is in tableB for the age and country pair. Is there some SQL query/queries I can do to add that new "id" column which is based on that of table B without having to use a cursor?

Answer 1

select a.a_name, a.age, a.country, b.id
FROM tableA a
JOIN tableB b on a.age = b.age and a.country = b.country

SqlFiddle

Answer 2

The previous answer is correct, but ignores a couple of problems in the way you are approaching the issue.

You should have your Country table referenced by a country_id field in your kids table: that way you are not duplicating the info (what happens in your design when a country changes its name?). You are also making a poor choice by storing the age of the people: you should store the date (or year) of birth instead, to avoid having to recalculate their age.

Kids table:

name | DOB         | country_id | id
Joe  | Aug 11 2005 | 1          | 1
Jim  | Sep 09 2007 | 21         | 2

Countries table:

id | name
1  | UK
2  | Germany

Then the select statement you need is:

select kids.name, kids.DOB, countries.name, kids.id from kids join countries on kids.country_id = countries.id

You can then either calculate the ages in your application, or you can incorporate a calculation into your SQL, see (for example) http://ma.tt/2003/12/calculate-age-in-mysql/

Edit

After reading your clarification that the age column is the age of the person at the time of their visit to the country , I would suggest a three-table design instead, still following the same principles of storing dates, not ages, and only storing each item of data once:

People table

Structure

CREATE TABLE `countries` (
 `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
 `name` varchar(22) NOT NULL DEFAULT '',
 PRIMARY KEY (`id`)
) ENGINE=InnoDB;

Sample content

id | name  | DOB
1  | Joe   | 2005-08-11
2  | Jim   | 2007-09-09
3  | Carol | 2008-01-22

Countries table

Structure

CREATE TABLE `countries` (
 `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
 `name` varchar(22) NOT NULL DEFAULT '',
 PRIMARY KEY (`id`)
) ENGINE=InnoDB;

Sample data

id | name
1  | UK
2  | Germany
3  | Saudi Arabia

Visits table

Structure

CREATE TABLE `visits` (
 `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
 `person_id` int(11) unsigned NOT NULL,
 `country_id` int(11) unsigned NOT NULL,
 `date_of_visit` date NOT NULL,
 PRIMARY KEY (`id`),
 KEY `person_id` (`person_id`),
 KEY `country_id` (`country_id`),
 CONSTRAINT `visits_ibfk_1` FOREIGN KEY (`person_id`) REFERENCES `people` (`id`),
 CONSTRAINT `visits_ibfk_2` FOREIGN KEY (`country_id`) REFERENCES `countries` (`id`)
) ENGINE=InnoDB;

Sample data

id | person_id | country_id | date_of_visit
1  | 1         | 1          | 2009-01-02
2  | 1         | 2          | 2010-01-01

Then the query you need to get the data you want is:

select 
 people.name, 
 people.DOB, 
 countries.name, 
 visits.date_of_visit 
from 
 visits 
  join 
 countries 
  on 
   visits.country_id = countries.id
  join 
 people
  on
   visits.person_id = people.id;