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.
This question is related to
mysql
sql
postgresql
Thanks for the help. This is what I ended up doing in order to make it work.
CREATE VIEW V AS
SELECT *
FROM ((POP NATURAL FULL OUTER JOIN FOOD)
NATURAL FULL OUTER JOIN INCOME);
This works too and you dont have to use join or anything:
DROP VIEW IF EXISTS yourview;
CREATE VIEW yourview AS
SELECT table1.column1,
table2.column2
FROM
table1, table2
WHERE table1.column1 = table2.column1;
Are you using MySQL or PostgreSQL?
You want to use JOIN syntax, not UNION. For example, using INNER JOIN:
CREATE VIEW V AS
SELECT POP.country, POP.year, POP.pop, FOOD.food, INCOME.income
FROM POP
INNER JOIN FOOD ON (POP.country=FOOD.country) AND (POP.year=FOOD.year)
INNER JOIN INCOME ON (POP.country=INCOME.country) AND (POP.year=INCOME.year)
However, this will only show results when each country and year are present in all three tables. If this is not what you want, look into left outer joins (using the same link above).
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/
Source: Stackoverflow.com