How to get the min value after an union in mysql? - mysql

Question

Let's say I have 2 tables A and B.
These 2 tables have 3 columns in common Name, Id and Price.
This is the query I used for 1 table :
SELECT Name, Id, Price FROM A WHERE Id = "123" and Price = (SELECT MIN(Price) FROM A);
I've just realised that this query doesn't work when the lowest price is held by another Id.
So I've look around and I think I should use GROUP BY ?
I've changed it to :
SELECT Name, MIN(Price) FROM A WHERE Id = "123" GROUP BY Name;
But this is not the expected result.
Let's say in table A I have :
Name
Id
Price
Au
123
12
Be
123
16
St
122
9
Ge
123
10
And for table B I have :
Name
Id
Price
La
123
14.5
La
123
12
St
123
13
Is
123
12
Is
123
10
La
123
10
Is
123
10
And the expected result is :
Name
Price
Ge
10
Is
10
La
10
The expected result is 1 row long because in the set of data there is only one row that match the condition but if I had another row with a Price of 10 and an Id of 123 it should be there also. So if there are more rows that matched the condition I want them in the result.
The problem is that when I do the following query using UNION I don't know how to get the lowest price for a specific Id:
SELECT Name, Id, Price FROM A UNION SELECT Name, Id, Price FROM B;
So what can I add to my query to have the expected result and then how would it work if I use union to get the lowest price of a specific Id over 2 tables ?

Answer 1

On MySQL 8+, we can use RANK here with a union query:
WITH cte AS (
SELECT Name, Id, Price FROM A WHERE Id = '123'
UNION ALL
SELECT Name, Id, Price FROM B WHERE Id = '123'
),
cte2 AS (
SELECT *, RANK() OVER (ORDER BY price) rnk
FROM cte
)
SELECT Name, Id, Price
FROM cte2
WHERE rnk = 1;
Here is a query which should work on earlier versions of MySQL:
SELECT Name, Id, Price
FROM
(
SELECT Name, Id, Price FROM A WHERE Id = '123'
UNION ALL
SELECT Name, Id, Price FROM B WHERE Id = '123'
) t
WHERE Price = (
SELECT Price FROM A WHERE Id = '123'
UNION ALL
SELECT Price FROM B WHERE Id = '123'
ORDER BY Price
LIMIT 1
);

Answer 2

This should be ok for you:
select *
from (select Name, id, Price FROM A
union
select Name, id, Price FROM B) Tab
where (Tab.id, Tab.price) = (select Tab2.id, min(Tab2.price)
from (select Name, id, Price FROM A
union
select Name, id, Price FROM B) Tab2
where Tab2.id = '123')
You have only one place where you put the ID you are looking for.
Here you can see the demo:
DEMO
/*This returns everything from your two tables*/
select *
from (SELECT Name, id, Price FROM A
union
select Name, id, Price FROM B) Tab
/*this returns the minimal price for your requested ID, here you requested id =123*/
select Tab2.id, min(Tab2.price)
from (SELECT Name, id, Price FROM A
union
select Name, id, Price FROM B) Tab2
where Tab2.id = '123'
--with this where clause:
where (Tab.id, Tab.price)
/*you are telling the query :
give me every row from all the data(first query)that has
this combination of ID + PRICE:
123 + 10 (you have found this with the second query)
So, you do not care what name it is, it only has to have :
ID = 123 and the lowest price which is 10.
ID 123 was requested from you and lowest price for that ID is 10,
which you have founded with the second query.*/

Answer 3

I tested this on db-fiddle.com and it returns all the rows with the lowest price:
SELECT Id, Name, Price
FROM (SELECT * FROM A UNION SELECT * FROM B) TMP
WHERE (Price, Id) = (
SELECT MIN(Price), Id
FROM (SELECT * FROM A UNION SELECT * FROM B) TMP2
WHERE Id = "123"
);
Here are the script for the tables I tested the query against:
create table A(
_id INT NOT NULL AUTO_INCREMENT,
Id VARCHAR(100) NOT NULL,
Name VARCHAR(100) NOT NULL,
Price INT NOT NULL,
PRIMARY KEY ( _id )
);
create table B(
_id INT NOT NULL AUTO_INCREMENT,
Id VARCHAR(100) NOT NULL,
Name VARCHAR(100) NOT NULL,
Price INT NOT NULL,
PRIMARY KEY ( _id )
);
INSERT INTO A(Id, Name, Price)
VALUES
('123', 'Name123a1', 21),
('123', 'Name123a2', 41),
('124', 'Name124a', 40);
INSERT INTO B(Id, Name, Price)
VALUES
('123', 'Name123b1', 22),
('123', 'Name123b2', 21),
('124', 'Name124b', 20);
The solution took some time to figure out, because I am rusty. Thanks to VBoka that helped me with sorting out bugs.

Answer 4

I would do this after your last query, the one that outputs 3 price values and you need the minimum.
create a cte
create a rank using ROW_NUMBER based on price in ASC order which is the default order if you want highest then add DESC in the end
filter the data with that rank column
with data as (
select
*, ROW_NUMBER () OVER(ORDER BY price ) as rank_ from table
)
select * from data where rank_ = 1