How can I make an intersection of two columns of two tables and get the entire rows?
Question:
I have a SQLite table. Let’s call it people.
Name
Age
Jane
50
John
80
Alice
46
Mark
25
Harry
33
I have another table work.
Name
work_id
Jane
1
Amanda
2
Filip
3
Alice
4
Jack
5
Harry
6
I’d like to get all rows whose name is in both people and work. But I do not only want the names. In fact I don’t care about the names at all. I just need them to find the matching entries. I want the work.work_id and people.age columns of the matching rows. The result should look like this:
work_id
age
1
50
4
46
6
33
Both tables can have hundreds to thousands of entries.
I also need a difference of the two i.e. The rows of work whose name isn’t in people. But this should be solvable with the second solution I have outlined below.
I am doing this in Python3 using the builtin sqlite3 module. But this should be a purely SQL problem independent of the SQL client.
What I’ve tried
The obvious choice is to do an INTERSECT:
SELECT Name FROM people INTERSECT SELECT Name FROM work_id
As I said, I need the Name columns to find the intersection of the tables but I need the rows themselves to get the things I actually want, people.age and work.work_id, not the Names.
The internet lead me to subqueries.
SELECT Name, Age FROM people where Name IN (SELECT Name FROM work)
This is a pretty powerful technique but I also need the work_id column of work so this isn’t a solution.
Is this comparing each row in people with all rows of work? Is the number of comparisons SELECT Count(*) FROM people × SELECT Count(*) FROM work or is it somehow optimized?
Answers:
You want to select columns from both tables and this means you need an INNER JOIN:
SELECT w.work_id,
p.Age
FROM work AS w INNER JOIN people AS p
ON p.Name = w.Name;
For the rows of work whose name isn’t in people use NOT IN:
SELECT *
FROM work
WHERE Name NOT IN (SELECT Name FROM people);
See the demo.
SELECT work_id, Age
FROM people AS p
JOIN work AS w ON p.Name = w.Name;
SELECT name, work_id
FROM work
EXCEPT
SELECT name
FROM people
I have a SQLite table. Let’s call it people.
| Name | Age |
|---|---|
| Jane | 50 |
| John | 80 |
| Alice | 46 |
| Mark | 25 |
| Harry | 33 |
I have another table work.
| Name | work_id |
|---|---|
| Jane | 1 |
| Amanda | 2 |
| Filip | 3 |
| Alice | 4 |
| Jack | 5 |
| Harry | 6 |
I’d like to get all rows whose name is in both people and work. But I do not only want the names. In fact I don’t care about the names at all. I just need them to find the matching entries. I want the work.work_id and people.age columns of the matching rows. The result should look like this:
| work_id | age |
|---|---|
| 1 | 50 |
| 4 | 46 |
| 6 | 33 |
Both tables can have hundreds to thousands of entries.
I also need a difference of the two i.e. The rows of work whose name isn’t in people. But this should be solvable with the second solution I have outlined below.
I am doing this in Python3 using the builtin sqlite3 module. But this should be a purely SQL problem independent of the SQL client.
What I’ve tried
The obvious choice is to do an INTERSECT:
SELECT Name FROM people INTERSECT SELECT Name FROM work_id
As I said, I need the Name columns to find the intersection of the tables but I need the rows themselves to get the things I actually want, people.age and work.work_id, not the Names.
The internet lead me to subqueries.
SELECT Name, Age FROM people where Name IN (SELECT Name FROM work)
This is a pretty powerful technique but I also need the work_id column of work so this isn’t a solution.
Is this comparing each row in people with all rows of work? Is the number of comparisons SELECT Count(*) FROM people × SELECT Count(*) FROM work or is it somehow optimized?
You want to select columns from both tables and this means you need an INNER JOIN:
SELECT w.work_id,
p.Age
FROM work AS w INNER JOIN people AS p
ON p.Name = w.Name;
For the rows of work whose name isn’t in people use NOT IN:
SELECT *
FROM work
WHERE Name NOT IN (SELECT Name FROM people);
See the demo.
SELECT work_id, Age
FROM people AS p
JOIN work AS w ON p.Name = w.Name;
SELECT name, work_id
FROM work
EXCEPT
SELECT name
FROM people