How to use variable from another table for UNION - mysql

Question

I have two sql commands I want to combine. I have changed the variables I am actually using in an attempt make it simpler to explain.
I would like to get the name of all fruits and vegetables with the colors that are a favoriteColor of everyone who's age is equal to the given value.
Currently I have these queries split up and I get the favorite color of people with SELECT favoriteColor FROM people WHERE age = ? and then I get all the fruits and vegetables where the color matches the favoriteColor of each person.
I get the matching fruits and vegetables like this:
SELECT * FROM ((SELECT 1 as type, name FROM fruits WHERE color = ?)
UNION ALL
(SELECT 2 as type, name FROM vegetables WHERE color = ?)) results
I basically want something like this, but I haven't been able to get it to work and I also do not want to have to run the same SELECT query twice:
SELECT * FROM ((SELECT 1 as type, name FROM fruits WHERE color =
(SELECT favoriteColor FROM people WHERE age = ?))
UNION ALL
(SELECT 2 as type, name FROM vegetables WHERE color =
(SELECT favoriteColor FROM people WHERE age = ?))) results
And I don't mind if I get duplicated fruits and vegetables, I need the duplicates for my situation.
For example:
If there are 2 people who are 30 years old and both of them like the color red, I want to get all fruits and vegetables that are red twice.
If there are 2 people who are 10 years old and one of them likes the color red and the other one also likes the color green, I want to get all fruits and vegetables that are red and green.

Answer 1

Not sure why you thought you had to test the colour in the union since the driver is people. And I have guessed what our output should be.
create table people(id int,name varchar(10),colour varchar(1),age int);
insert into fruits values
(1,'a','a'),(2,'b','a'),(3,'b','b'),(4,'b','c');
insert into vegetables values
(1,'t','a'),(2,'t','u'),(3,'v','v'),(4,'v','w');
insert into people values
(1,'aa','a',10),(2,'bb','b',10),(3,'cc','c',10),(4,'dd','c',11);
select p.name,p.age,p.name,s.`type`,s.name,s.colour
from people p
join
(
select 1 as type, name,colour from fruits
union
select 2 as type, name,colour from vegetables
) s
on s.colour = p.colour
where p.age = 10;
+------+------+------+------+------+--------+
| name | age | name | type | name | colour |
+------+------+------+------+------+--------+
| aa | 10 | aa | 2 | t | a |
| aa | 10 | aa | 1 | b | a |
| aa | 10 | aa | 1 | a | a |
| bb | 10 | bb | 1 | b | b |
| cc | 10 | cc | 1 | b | c |
+------+------+------+------+------+--------+
5 rows in set (0.00 sec)

Answer 2

i don't know but the simplied version may be
SELECT * FROM ((SELECT 1 as type, name, color FROM fruits WHERE color = ?) UNION ALL
(SELECT 2 as type, name, color FROM vegetables WHERE color = ?)) results
where results.color= (SELECT favoriteColor FROM people WHERE age = ?)
sorry for indentation

Answer 3

I'd do it as a pair of unions to create one unified dataset, joined to another (then filtered) dataset:
SELECT * FROM
(
SELECT 1 as type, name, color FROM fruits
UNION ALL
SELECT 2 as type, name, color FROM vegetables
) plants pl
INNER JOIN
people pe ON pl.color = pe.favoriteColor
WHERE
pe.age = 30
If you want different columns out of fruit and veg, and there might not be a fruit or veg row for a given color:
SELECT * FROM
people pe
LEFT JOIN fruits f on pe.favoriteColor = f.color
LEFT JOIN veg v on pe.favoriteColor = f.color
WHERE
pe.age = 30
But bear in mind that multiple fruits or veg of a given color will cause the result set to multiply in duplicate for the other plant, which could become a nightmare to deal with on the front end