How to retrieve rows in column with highest value using SQL [duplicate]

Question

I have a table of documents (here is a simplified version):

id	change	content
1	1	...
2	1	...
1	2	...
1	3	...

How to select one row for each id and only select the largest rev?

Based on the above data, the result should contain two rows: [1, 3, ...] and [2, 1, ..]. I'm using MySQL.

Currently, I'm using a check in a while loop to detect and overwrite the old rev in the result set. But is this the only way to achieve results? Is there no SQL solution?

Answer 1

I prefer to use as little code as possible...

You can use IN to achieve Try this:

SELECT * 
FROM t1 WHERE (id,rev) IN 
( SELECT id, MAX(rev)
  FROM t1
  GROUP BY id
)

In my opinion, this is simpler... easier to read and maintain.

Answer 2

At first glance...

You only need to use the MAX aggregate function in the GROUP BY clause:

SELECT id, MAX(rev)
FROM YourTable
GROUP BY id

Things are never that simple, right?

I just noticed that you also need the content column.

In SQL, this is a very common problem: find the entire row of data with the maximum value in a certain column based on a certain grouping identifier. I've heard this question a lot in my career. In fact, this is a question I answered during a technical interview for my current job.

This question is actually so common that the Stack Overflow community created a tag specifically to deal with this type of question: greatest-n-per-group.

Basically, you have two ways to solve this problem:

Use simple group-identifier, max-value-in-groupSubquery to connect

In this approach, you first find the group-identifier, max-value-in-group (already solved above) in a subquery. You then join your table with the subquery, using group-identifier and max-value-in-group for an equijoin:

SELECT a.id, a.rev, a.contents
FROM YourTable a
INNER JOIN (
    SELECT id, MAX(rev) rev
    FROM YourTable
    GROUP BY id
) b ON a.id = b.id AND a.rev = b.rev

Use self-join for left join, and adjust the connection conditions and filtering conditions

In this approach, you left join the table to itself. Equivalent connections are placed in group-identifier. Then, there are two clever steps:

1. The second connection condition is that the value on the left is less than the value on the right
2. When you do step 1, the row that actually has the largest value will have NULL on the right (remember this is a LEFT JOIN). We then filter the results of the join to only show rows with NULL on the right.

So, you end up with:

SELECT a.*
FROM YourTable a
LEFT OUTER JOIN YourTable b
    ON a.id = b.id AND a.rev < b.rev
WHERE b.id IS NULL;

in conclusion

Both methods will give exactly the same results.

If there are two rows with max-value-in-group in group-identifier, then these two rows will appear in the result in both methods.

Both methods are SQL ANSI compatible, so no matter what "flavor" of RDBMS you prefer, you can use it.

Both methods are also very friendly in terms of performance, but your actual situation may be different (RDBMS, database structure, index, etc.). Therefore, benchmark when choosing a method. Make sure to choose the method that makes the most sense for you.