Using SQL Server 2014:
Consider the following table:
DECLARE @Table TABLE (
Id int NOT NULL identity(1,1),
Col_Value varchar(2)
)
INSERT INTO @Table (Col_Value)
VALUES ('A'),('A'),('B'),('B'),('B'),('A'),('A'),('B'),('B'),('B'),('A'),('B'),('B'),('A'),('A'),('B'),('C'),('C'),('A'),('A'),('B'),('B'),('C')
How can I create a query that produces R column in the result like below
+----+------+---+
| ID | Data | R |
+----+------+---+
| 1 | A | 1 |
+----+------+---+
| 2 | A | 2 |
+----+------+---+
| 3 | B | 1 |
+----+------+---+
| 4 | B | 2 |
+----+------+---+
| 5 | B | 3 |
+----+------+---+
| 6 | A | 1 |
+----+------+---+
| 7 | A | 2 |
+----+------+---+
| 8 | B | 1 |
+----+------+---+
| 9 | B | 2 |
+----+------+---+
| 10 | B | 3 |
+----+------+---+
| 11 | A | 1 |
+----+------+---+
| 12 | B | 1 |
+----+------+---+
| 13 | B | 2 |
+----+------+---+
| 14 | A | 1 |
+----+------+---+
| 15 | A | 2 |
+----+------+---+
| 16 | B | 1 |
+----+------+---+
| 17 | C | 1 |
+----+------+---+
| 18 | C | 2 |
+----+------+---+
| 19 | A | 1 |
+----+------+---+
| 20 | A | 2 |
+----+------+---+
| 21 | B | 1 |
+----+------+---+
| 22 | B | 2 |
+----+------+---+
| 23 | C | 1 |
+----+------+---+
In the above result table, once Data column changes in a row, the R value resets to 1
Update 1
Ben Thul's answer works very well.
I suggest below post be updated with a reference to this answer.