Selecting only distinct rows based on a column in Knex

Question

I'm using Knex , a pretty nice SQL builder.

I've got a table called Foo which has 3 columns

+--------------+-----------------+
| id           | PK              |
+--------------+-----------------+
| idFoo        | FK (not unique) |
+--------------+-----------------+
| serialNumber | Number          |
+--------------+-----------------+

I'd like to select all rows with idFoo IN (1, 2, 3) .

However I'd like to avoid duplicate records based on the same idFoo .

Since that column is not unique there could be many rows with the same idFoo .

A possible solution

My query above will of course return all with idFoo IN (1, 2, 3) , even duplicates.

db.select(
  "id",
  "idFoo",
  "age"
)
.from("foo")
.whereIn("idFoo", [1, 2, 3])

However this will return results with duplicated idFoo 's like so:

+----+-------+--------------+
| id | idFoo | serialNumber |
+----+-------+--------------+
| 1  | 2     | 56454        |
+----+-------+--------------+
| 2  | 3     | 75757        |
+----+-------+--------------+
| 3  | 3     | 00909        |
+----+-------+--------------+
| 4  | 1     | 64421        |
+----+-------+--------------+

What I need is this:

+----+-------+--------------+
| id | idFoo | serialNumber |
+----+-------+--------------+
| 1  | 2     | 56454        |
+----+-------+--------------+
| 3  | 3     | 00909        |
+----+-------+--------------+
| 4  | 1     | 64421        |
+----+-------+--------------+

I can take the result and use Javascript to filter out the duplicates. I'd specifically like to avoid that and write this in Knex.

The question is how can I do this with Knex code?

I know it can be done with plain SQL (perhaps something using GROUP BY ) but I'd specifically like to achieve this in "pure" knex without using raw SQL.

Answer 1

In normal sql you do it like this.

You perform a self join and try to find a row with same idFoo but bigger id , if you dont find it you have NULL . And will know you are the bigger one.

 SELECT t1.id, t1.idFoo, t1.serialNumber
 FROM foo as t1
 LEFT JOIN foo as t2
   ON t1.id < t2.id
  AND t1.idFoo = t2.idFoo  
 WHERE t2.idFoo IS NULL

So check for left join on knex.js

EDIT:

Just check documentation build this (not tested):

 knex.select('t1.*')
     .from('foo as t1')
     .leftJoin('foo as t2', function() {
        this.on('t1.id', '<', 't2.id')
            .andOn('t1.idFoo ', '=', 't2.idFoo')
        })
     .whereNull("t2.idFoo") 

Answer 2

Knex.js supports groupBy natively. You can write:

knex('foo').whereIn('id',
  knex('foo').max('id').groupBy('idFoo')
)

Which is rewritten to the following SQL:

SELECT * FROM foo
  WHERE id IN (
    SELECT max(id) FROM foo
      GROUP BY idFoo
  )

Note that you need to use the subselect to make sure you won't mix values from diffrent rows within the same group.