SQL DISTINCT is not a function

Posted on by lukaseder
A very common misconception I often encounter with SQL users is the idea that DISTINCT is something like a function, and that it can take parenthesised arguments. Just recently, I’ve seen this Stack Overflow question where the OP was looking for a way to express this in jOOQ: 

SELECT DISTINCT (emp.id), emp.fname, emp.name
FROM employee emp;
Notice the parentheses around (emp.id), which look as though this is some special kind of DISTINCT usage, which is akin to a DISTINCT function. The idea is often that:
  • The behaviour is somewhat different from omitting the parentheses
  • The performance is faster, because only the ID needs to be considered for distinctness

This is incorrect

These claims are incorrect, of course. There is no semantic or performance difference between the two. The parentheses are merely parentheses around a column expression, in a similar way as you would use parentheses to influence operator precedence. Think of it this way: 

SELECT DISTINCT (emp.id + 1) * 2, emp.fname, emp.name
FROM employee emp;
In the above example, we do not apply a “DISTINCT function” to the expression emp.id + 1. We merely placed parentheses around a column expression emp.id + 1 to make sure the addition happens before the multiplication. The DISTINCT operator happens after the projection, always. If SQL had used a more logical syntax, rather than following English grammar (it was originally called Structured English QUEry Language, or SEQUEL), then we would write the OP’s statement like this: 

FROM employee
SELECT id, fname, name
DISTINCT
Again, the DISTINCT operation always happens after the projection (SELECT clause content), and is applied to the entirety of the projection. There is no way in standard SQL to apply distinctness only to parts of the projection (there is in PostgreSQL, see further down). To clarify this a bit more, I recommend reading our previous blog post about the logical order of operations in SQL, and how DISTINCT and ORDER BY are related.

What would it mean anyway?

We can revert the question back to the OP and ask ourselves, what would it mean for a DISTINCT operation to apply to only one column, anyway? Let’s assume this data set: 
|id |fname|name|
|---|-----|----|
|1  |A    |A   |
|1  |B    |B   |
If we applied DISTINCT only to the ID column (and didn’t project anything else), clearly, we’d only get one row as a result: 

SELECT DISTINCT id FROM employee


|id |
|---|
|1  |
But if we wanted to also project FNAME and NAME, which row would “win”? Would we display the first or the second row, or any random row? The behaviour would be undefined, and SQL doesn’t like undefined behaviour, so this is not possible. The only reasonable application of DISTINCT is always on the entire projection.

Exception: PostgreSQL

Fortunately (or to add more to syntactic confusion: unfortunately), PostgreSQL has implemented an extension to the SQL standard. With DISTINCT ON, it is effectively possible to apply distinctness only to parts of the projection: 

WITH emp (id, fname, name) AS (
  VALUES (1, 'A', 'A'),
         (1, 'B', 'B')
)
SELECT DISTINCT ON (id) id, fname, name
FROM emp
ORDER BY id, fname, name
The output is now what the OP desired (but couldn’t use, because they were using MySQL): 
|id         |fname|name |
|-----------|-----|-----|
|1          |A    |A    |
I personally don’t like DISTINCT ON. While it is very useful, no doubt, it makes something that is already very difficult to explain to SQL beginners even more complicated. With a “more reasonable” syntax, the query would be written like this: 

FROM emp
SELECT id, fname, name
ORDER BY id, fname, name
DISTINCT ON (id)
With this syntactic order of operation, there would be no doubt about the semantics of DISTINCT or DISTINCT ON.

Published by lukaseder

I made jOOQ

7 thoughts on “SQL DISTINCT is not a function”

  1. We used to have both SELECT DISTINCT and SELECT ALL operators. The latter one defaulted to the SELECT that we use today.

    Reply

  2. The standard GROUP BY clause can express this, but if the other selected columns are not already aggregate expressions then something like MIN() is needed to make a clear winner from multiple different names for each id.

    Reply

    1. That’s treacherous, because you’ll get a per-column MIN() evaluation not a per-row. For example, if your data is this:

      +------+------+------+
| col1 | col2 | col3 |
+------+------+------+
|    1 | a    | b    |
|    1 | b    | a    |
+------+------+------+

      Your approach would lead to this result, which is probably not desireable:

      +------+------+------+
| col1 | col2 | col3 |
+------+------+------+
|    1 | a    | a    |
+------+------+------+
      Reply

  3. I am also not a fan of DISTINCT ON (…). ORDER BY. I prefer to think about such cases in terms of filtered ranking function:

    -- Snowflake/Teradata
SELECT *
FROM VALUES (1, 'A', 'A'), (1, 'B', 'B') emp(id, fname, name) 
QUALIFY ROW_NUMBER() OVER(PARTITION BY id ORDER BY fname, name) = 1;
    Reply

    1. You folks are spoiled with that QUALIFY clause. I really wish this became part of the standard. It makes a lot of sense, too

      Reply

  4. DISTINCT and DISTINCT ON are often a bad code smell as I’ve experienced them in production code, as it’s common to take a short cut toward the real constraints using it.

    GROUP BY and thoroughly modern fun like windowing functions often signify of more complete working out of the constraints.

    This isn’t to say there’s never a place for it, just that when you see it, it’s a signal to examine carefully.

    Reply

    1. I can see DISTINCT being a sign of this problem, but DISTINCT ON? The clause is sophisticated and complicated enough that I would not have expected it to be used as much by people who don’t model their databases properly…

      Reply

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