The
SQL:2003 standard ranking functions are awesome companions and useful tools every now and then. The ones that are supported in almost all databases are:
- ROW_NUMBER(): This one generates a new row number for every row, regardless of duplicates within a partition.
- RANK(): This one generates a new row number for every distinct row, leaving gaps between groups of duplicates within a partition.
- DENSE_RANK(): This one generates a new row number for every distinct row, leaving no gaps between groups of duplicates within a partition.
As always, the above is much easier to understand by example. Let’s assume the following PostgreSQL schema containing a table with 8 records, some of which are duplicates:
CREATE TABLE t AS
SELECT 'a' v UNION ALL
SELECT 'a' UNION ALL
SELECT 'a' UNION ALL
SELECT 'b' UNION ALL
SELECT 'c' UNION ALL
SELECT 'c' UNION ALL
SELECT 'd' UNION ALL
SELECT 'e'
Now, let’s select each value, along with the three aforementioned ranking functions. And for kicks and giggles, we’ll use the SQL standard
WINDOW clause! Yay, it saved us 15 characters of repetitive SQL code. Note that the WINDOW clause is hardly implemented apart by PostgreSQL and Sybase SQL Anywhere…
SELECT
v,
ROW_NUMBER() OVER (window) row_number,
RANK() OVER (window) rank,
DENSE_RANK() OVER (window) dense_rank
FROM t
WINDOW window AS (ORDER BY v)
ORDER BY v
And the above results in:
+---+------------+------+------------+
| V | ROW_NUMBER | RANK | DENSE_RANK |
+---+------------+------+------------+
| a | 1 | 1 | 1 |
| a | 2 | 1 | 1 |
| a | 3 | 1 | 1 |
| b | 4 | 4 | 2 |
| c | 5 | 5 | 3 |
| c | 6 | 5 | 3 |
| d | 7 | 7 | 4 |
| e | 8 | 8 | 5 |
+---+------------+------+------------+
(See also this SQLFiddle)
How DENSE_RANK() can help when writing SELECT DISTINCT
No doubt,
ROW_NUMBER() is the most useful ranking function among the above, specifically when you need to emulate LIMIT .. OFFSET clauses as in DB2, Oracle (11g or less), Sybase SQL Anywhere (prior to version 12), SQL Server (2008 and less).
Read on here about how
jOOQ emulates this SQL clause in various SQL dialects.
But using
ROW_NUMBER() has a subtle problem when used along with
DISTINCT or
UNION. It prevents the database from being able to remove duplicates, because
ROW_NUMBER will always produce distinct values within a partition. In the above example, duplicate values for
T.V were added on purpose. How can we first remove the duplicates and only then enumerate row numbers? Clearly, we can no longer use
ROW_NUMBER(). The following query:
SELECT DISTINCT
v,
ROW_NUMBER() OVER (window) row_number
FROM t
WINDOW window AS (ORDER BY v)
ORDER BY v, row_number
… yields
+---+------------+
| V | ROW_NUMBER |
+---+------------+
| a | 1 |
| a | 2 |
| a | 3 |
| b | 4 |
| c | 5 |
| c | 6 |
| d | 7 |
| e | 8 |
+---+------------+
(See also this SQLFiddle)
But we can use
DENSE_RANK(), instead! With
DENSE_RANK() the ranking is applied in a way that duplicate records will receive the same rank. And there are no gaps between ranks. Hence:
SELECT DISTINCT
v,
DENSE_RANK() OVER (window) row_number
FROM t
WINDOW window AS (ORDER BY v)
ORDER BY v, row_number
… which yields:
+---+------------+
| V | ROW_NUMBER |
+---+------------+
| a | 1 |
| b | 2 |
| c | 3 |
| d | 4 |
| e | 5 |
+---+------------+
(See also this SQLFiddle)
Thus, remember…
Thus, remember: ROW_NUMBER() is to SELECT what DENSE_RANK() is to SELECT DISTINCT 
Caveats
In order for the above to be true, however, you must ensure that all expressions from the
SELECT DISTINCT clause are used in the
DENSE_RANK()‘s
OVER(ORDER BY ...) clause. For example:
SELECT DISTINCT
v1,
v2,
v3,
DENSE_RANK() OVER (window) row_number
FROM t
WINDOW window AS (ORDER BY v1, v2, v3)
If any of
v1, v2, v3 are other ranking functions or aggregate functions, or non-deterministic expressions, etc., the above trick won’t work. But it’s still a nice trick to keep up one’s sleeves for the odd corner-case query, where distinct rows need row numbers.
And if you don’t want to think about this trick all the time, do note that we’ve baked this into
jOOQ, because we think that you should worry about business logic, not about SQL standardiation:
Like this:
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I have a input like
i need to pick the most top 3 played movie , i.e,
please help
That’s not really a query where you need window functions. It can be solved easily with SQL-92 aggregate functions, which run on any database:
Note that a great platform to ask these questions is Stack Overflow
select top 3 count(movie_name) total, movie_name from movies
group by movie_name
order by count(movie_name) desc;
with cteMovie as
(
select movie_name, played_time, row_number() over (partition by movie_name order by played_time desc ) as number from my_table
)
select * from cteMovie where number=1
Thank you for your contribution. Yes, that is how you can filter by such a row number. In what context did you think this fits the article at hand?
How to use the “where” function for row_number
for example:
row-number > 3
You’ll have to calculate the window function value in a derived table:
SELECT * FROM ( SELECT row_number() OVER ( ... ) AS rn FROM ... ) t WHERE t.rn > 3can any one please post the query to find the 10th highest record in HIVE.
Sounds like an excellent question for Stack Overflow! https://stackoverflow.com
We are sybase database, dont have DENSE_RANK, PARTITION BY functionality. How to achieve the DENSE_RANK() OVER (PARTITION BY with out using In-built functions.
You migrate off Sybase ;) Cheers
On a more serious note: DENSE_RANK() can be emulated with an ordinary subquery.
is the same as:
SELECT o.*, ( SELECT COUNT(DISTINCT y) FROM t AS i WHERE i.x = o.x AND i.y < o.y ) AS dense_rank FROM t AS oChances are, that this query is quite slow, though.
i want to generate rank order by 2 columns.Is there is any solution for that
Just
RANK() OVER (ORDER BY col1, col2)