Only select first row of repeating value in a column in SQL - sql

Question

I have table that has a column that may have same values in a burst. Like this:
+----+---------+
| id | Col1 |
+----+---------+
| 1 | 6050000 |
+----+---------+
| 2 | 6050000 |
+----+---------+
| 3 | 6050000 |
+----+---------+
| 4 | 6060000 |
+----+---------+
| 5 | 6060000 |
+----+---------+
| 6 | 6060000 |
+----+---------+
| 7 | 6060000 |
+----+---------+
| 8 | 6060000 |
+----+---------+
| 9 | 6050000 |
+----+---------+
| 10 | 6000000 |
+----+---------+
| 11 | 6000000 |
+----+---------+
Now I want to prune rows where the value of Col1 is repeated and only select the first occurrence.
For the above table the result should be:
+----+---------+
| id | Col1 |
+----+---------+
| 1 | 6050000 |
+----+---------+
| 4 | 6060000 |
+----+---------+
| 9 | 6050000 |
+----+---------+
| 10 | 6000000 |
+----+---------+
How can I do this in SQL?
Note that only burst rows should be removed and values can be repeated in non-burst rows! id=1 & id=9 are repeated in sample result.
EDIT:
I achieved it using this:
select id,col1 from data as d1
where not exists (
Select id from data as d2
where d2.id=d1.id-1 and d1.col1=d2.col1 order by id limit 1)
But this only works when ids are sequential. With gaps between ids (deleted ones) the query breaks. How can I fix this?

Answer 1

You can use a EXISTS semi-join to identify candidates:
Select wanted rows:
SELECT * FROM tbl t
WHERE NOT EXISTS (
SELECT *
FROM tbl
WHERE col1 = t.col1
AND id = t.id - 1
)
ORDER BY id;
Get rid of unwanted rows:
DELETE FROM tbl AS t
-- SELECT * FROM tbl t -- check first?
WHERE EXISTS (
SELECT *
FROM tbl
WHERE col1 = t.col1
AND id = t.id - 1
);
This effectively deletes every row, where the preceding row has the same value in col1, thereby arriving at your set goal: only the first row of every burst survives.
I left the commented SELECT statement because you should always check what is going to be deleted before you do the deed.
Solution for non-sequential IDs:
If your RDBMS supports CTEs and window functions (like PostgreSQL, Oracle, SQL Server, ... but not SQLite prior to v3.25, MS Access or MySQL prior to v8.0.1), there is an elegant way:
WITH cte AS (
SELECT *, row_number() OVER (ORDER BY id) AS rn
FROM tbl
)
SELECT id, col1
FROM cte c
WHERE NOT EXISTS (
SELECT *
FROM cte
WHERE col1 = c.col1
AND rn = c.rn - 1
)
ORDER BY id;
Another way doing the job without those niceties (should work for you):
SELECT id, col1
FROM tbl t
WHERE (
SELECT col1 = t.col1
FROM tbl
WHERE id < t.id
ORDER BY id DESC
LIMIT 1) IS NOT TRUE
ORDER BY id;

Answer 2

select min(id), Col1 from tableName group by Col1

Answer 3

If your RDBMS supports Window Aggregate functions and/or LEAD() and LAG() functions you can leverage them to accomplish what you are trying to report. The following SQL will help get you started down the right path:
SELECT id
, Col AS CurCol
, MAX(Col)
OVER(ORDER BY id ROWS BETWEEN 1 PRECEDING AND 1 PRECEDING) AS PrevCol
, MIN(COL)
OVER(ORDER BY id ROWS BETWEEN 1 FOLLOWING AND 1 FOLLOWING) AS NextCol
FROM MyTable
From there you can put that SQL in a derived table with some CASE logic that if the NextCol or PrevCol is the same as CurCol then set CurCol = NULL. Then you can collapse eliminate all the id records CurCol IS NULL.
If you don't have the ability to use window aggregates or LEAD/LAG functions your task is a little more complex.
Hope this helps.

Answer 4

Since id is always sequential, with no gaps or repetitions, as per your comment, you could use the following method:
SELECT t1.*
FROM atable t1
LEFT JOIN atable t2 ON t1.id = t2.id + 1 AND t1.Col1 = t2.Col1
WHERE t2.id IS NULL
The table is (outer-)joined to itself on the condition that the left side's id is one greater than the right side's and their Col1 values are identical. In other words, the condition is ‘the previous row contains the same Col1 value as the current row’. If there's no match on the right, then the current record should be selected.
UPDATE
To account for non-sequential ids (which, however, are assumed to be unique and defining the order of changes of Col1), you could also try the following query:
SELECT t1.*
FROM atable t1
LEFT JOIN atable t2 ON t1.id > t2.id
LEFT JOIN atable t3 ON t1.id > t3.id AND t3.id > t2.id
WHERE t3.id IS NULL
AND (t2.id IS NULL OR t2.Col1 <> t1.Col1)
The third self-join is there to ensure that the second one yields the row directly preceding that of t1. That is, if there's no match for t3, then either t2 contains the preceding row or it's got no match either, the latter meaning that t1's current row is the top one.