mysql - SQL - Create view from multiple tables

Question

I have three tables:

POP(country, year, pop)
FOOD(country, year, food)
INCOME(country, year, income)

I am trying to create a view such as:

V(country, year, pop, food, income)

This is my code so far. I don't think its correct:

CREATE VIEW V AS
(SELECT * FROM POP
UNION
SELECT * FROM FOOD
UNION
SELECT * FROM INCOME);

One issue is, a country that is present in POP may not be present in FOOD. Same goes for year. A year that is present in POP may not be present in FOOD. So, I keep thinking that the above code will not work.

Answer 1

Union is not what you want. You want to use joins to create single rows. It's a little unclear what constitutes a unique row in your tables and how they really relate to each other and it's also unclear if one table will have rows for every country in every year. But I think this will work:

CREATE VIEW V AS (

  SELECT i.country,i.year,p.pop,f.food,i.income FROM
    INCOME i
  LEFT JOIN 
    POP p 
  ON
    i.country=p.country
  LEFT JOIN
    Food f
  ON 
    i.country=f.country
  WHERE 
    i.year=p.year
  AND
    i.year=f.year
);

The left (outer) join will return rows from the first table even if there are no matches in the second. I've written this assuming you would have a row for every country for every year in the income table. If you don't things get a bit hairy as MySQL does not have built in support for FULL OUTER JOINs last I checked. There are ways to simulate it, and they would involve unions. This article goes into some depth on the subject: http://www.xaprb.com/blog/2006/05/26/how-to-write-full-outer-join-in-mysql/